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Forage nutritive value and steer responses to grazing intensity and seed-head suppression of endophyte-free tall fescue in mixed pastures1
The Professional Animal Scientist 31 (2015):120–129; http://dx.doi.org/10.15232/pas.2014-01366 ©2015 American Registry of Professional Animal Scientists
Forage nutritive value and steer responses to grazing intensity and seed-head suppression of endophyte-free tall fescue in mixed pastures1
B. M. Goff,* G. E. Aiken,†2 PAS, W. W. Witt,* P. L. Burch,‡ and F. N. Schrick§ *Department of Plant and Soil Sciences, University of Kentucky, Lexington 40546; †USDA-ARS, Forage and Animal Production Research Unit, Lexington, KY 40546; ‡P. L. Dow AgroSciences, Christiansburg, VA 24073; and §Department of Animal Sciences, University of Tennessee, Knoxville 37996-4574
ABSTRACT A 2-yr grazing experiment was conducted with 8- to 10-mo old steers on pastures of endophyte-free tall fescue [Lolium arundinaceum (Schreb.) Darbysh.] in mixture with other grasses to assess the effect of seed-head suppression of fescue on steer performance and forage nutritive values. Treatments with and without seed-head suppression were each combined with either light or moderate grazing intensities for assignment to twelve 1.0-ha pastures of the grass mixtures. The experiment was conducted as a randomized complete block design with 3 replications. Steer ADG was measured, and CP and in vitro DM digest-
1 Mention of trade names or commercial products in the article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA. The USDA is an equal opportunity employer. 2 Corresponding author: glen.aiken@ars. usda.gov
ibility (IVDMD) of available forage, and leaf blade and sheaths of fescue tillers, were monitored. Averaged over grazing intensities, ADG was 16% greater (P < 0.05) with seed-head suppression; however, a lower (P < 0.001) mean stocking rate with seed-head suppression resulted in a tendency of greater (P = 0.068) BW gain per hectare without suppression. Crude protein of available forage was consistently greater (P < 0.01) with seed-head suppression across all dates, whereas IVDMD was consistently greater (P < 0.01) with seed-head suppression in the late grazing season. Crude protein in leaf blades and sheaths of vegetative tillers with seed-head suppression were consistently greater (P < 0.01) than vegetative tillers without seed-head suppression. The IVDMD of blades and sheaths was similar (P > 0.18) between suppressed and nonsuppressed vegetative tillers, and both had greater (P < 0.05) IVDMD than reproductive tillers over most dates. Results showed seed-head suppression of fescue to improve steer ADG by increasing CP in vegetative tissues and improving digestibility of avail-
able forage by alleviating lower-quality, reproductive tillers. Key words: beef cattle, forage quality, plant growth regulation, seed-head suppression, tall fescue
INTRODUCTION Tall fescue [Lolium arundinaceum (Schreb.) Darbysh.] is a cool-season perennial grass that is used as a forage on approximately 14 million ha in the humid east (Thompson et al., 2001). Tall fescue is persistent and productive, but cattle production and thriftiness is low (Hoveland et al., 1980) because of toxic ergot alkaloids produced by a fungal endophyte (Neotyphodium coenophialum) that infects most plants of tall fescue (Bacon et al., 1977). Cattle that consume endophyte-infected tall fescue can undergo a toxicosis that causes cattle to exhibit poor weight gain, rough hair coats during the summer months, elevated core body temperatures, and reduced serum prolactin concentra-
Seed-head suppression of tall fescue
tions (Schmidt and Osborn, 1993; Strickland et al., 1993). Removal of fescue seed heads can be an effective strategy for reducing dosage of alkaloids by grazing animals because ergot alkaloid concentrations are greater in seeds than other plant parts (Rottinghaus et al., 1991) and cattle readily consume seed heads (Goff et al., 2012). Seed heads can be removed by mowing, but this is generally ineffective if done during seed maturation because cattle selectively graze seed heads over a short period during early seed development (Goff et al., 2012). Research demonstrated that the plant growth regulator mefluidide inhibits floral development of cool-season perennial grasses (Roberts and Moore, 1990; Moyer and Kelley, 1995) to maintain vegetative stands and improve nutritive value (Sheaffer and Marten, 1986), but mefluidide was not licensed for forages. Aiken et al. (2012) reported that steer ADG increased and signs of fescue toxicosis were mitigated on chemically seed head–suppressed tall fescue that was grazed; however, it could not be concluded whether the increase in weight gain was due to alleviation of toxic seed heads or by enhancement of nutritive value by maintaining fescue in the vegetative stage of growth. A follow-up grazing experiment was designed to use endophyte-infected fescue to evaluate the interaction between seed-head suppression and grazing intensity on steer performance and pasture responses. However, these pastures were inadvertently planted with endophyte-free seed. Nonetheless, this provided pastures of endophyte-free fescue that removed the ergot alkaloid effect and provided an objective evaluation of steer and forage nutritive-value responses to chemical seed-head suppression and light and moderate grazing intensities.
MATERIALS AND METHODS Experimental Site and Design The study was conducted at the University of Kentucky C. Oran Little Research Center, near Versailles, Ken-
tucky, on a McAfee silt loam (fine, mixed, active, mesic Mollic Hapludalf) and a Maury-Bluegrass silt loam (fine, mixed, active mixed, mesic Typic Paleudalf) complex. Twelve 1.0-ha pastures were sprayed twice with glyphosate (5 L/ha) and no-till planted with endophyte-free Kentucky 31 tall fescue on 19 March 2010 with 28 kg of pure live seed per hectare. Areas that exhibited poor emergence or losses of plants from intensive spot grazing were reseeded on 14 September 2011 at the same rate. Pastures were blocked according to the average slope and soil type. Two grazing intensities (light and moderate) and 2 rates of herbicide application were arranged as a 2 × 2 factorial with 3 replications. Herbicide treatments consisted of Chaparral {Dow AgroSciences, Indianapolis, IN; 62.13% aminopyralid [4-amino-3,6-dichloro-2-pyridinecarboxylic acid] and 9.45% metsulfuron-methyl [methyl 2-[[[[(4-methoxy-6-methyl-1,3,5triazin-2-yl)amino]carbonyl]amino] sulfonyl]benzoate]} applied at 0 or 140 g/ha (metsulfuron: 13.2 g/ha, aminopyralid: 87.0 g/ha) on 7 April 2011 and 19 March 2012. Control pastures received Milestone (DowAgrosciences LLC, Indianapolis, IN; 40.6% aminopyralid) at 220 g/ha (aminopyralid: 89.3 g/ha) on the same dates. All herbicides were applied with nonionic surfactant (0.25% vol/vol). Pastures were fertilized annually in mid-March with 78 kg of N/ha.
Steer Responses Steers used in 2010 were primarily crossbred Angus with initial BW of 263 ± 19 kg, and those used in 2011 were predominately Hereford, Charolais, and Angus with initial BW of 314 ± 11 kg in 2011. Tester steers were blocked by BW in 2010 and by BW and breed type in 2011 for assignment to pastures such that mean BW were similar across pastures and breed types. On the day that grazing was initiated, all steers were dewormed using moxidectin (Cydectin; Fort Dodge Animal Health, Fort Dodge, IA) and received steroidal
121 implants (Synovex; 200 mg of progesterone, 20 mg of estradiol; Fort Dodge Animal Health). Steers on pasture were provided free-choice trace minerals (zinc, 0.35% minimum; manganese, 0.2% minimum; iron, 0.2% minimum; copper, 0.03% minimum; selenium, 0.009% maximum; iodine, 0.007% minimum; cobalt, 0.005% minimum). Steers were maintained according to University of Kentucky Institutional Animal Care and Use Committee (IACUC) protocol #2011– 0797. Steers grazed the pastures for 74 d (5 May to 14 July) and 84 d (3 April to 26 June) for the 2011 and 2012 growing seasons, respectively. Three tester steers were used in each pasture, and put-and-take steers were used to provide light and moderate grazing intensities. Grazing intensities were established and monitored using a disk meter, similar in design to one described by Bransby et al. (1977), with the exception that the falling plate weighed 1.9 kg and had a diameter of 45 cm. Based on previous research (Aiken et al., 2012) and experience measuring forage mass with disk meters for seed head–suppressed and nonsuppressed pastures, mean disk meter heights (DMH) of 7 to 9 cm were targeted for the light grazing and 5 to 7 cm for the moderate intensity. Fifty DMH were randomly measured for each pasture at approximately 2-wk intervals. Stocking adjustments were made following the 2-wk measurements of DMH. Steers were weighed following a 12to 14-h fast at the start and end of each grazing season. Carcass-related traits of the tester steers were determined at the end of the grazing season using an Aloka SSD-500V (Hitachi Aloka Medical Ltd., Tokyo, Japan) ultrasound with a 3.5-MHz linear array transducer (UST 6049). Scans were taken between the 12th and 13th ribs to determine the longissimus dorsi area and backfat thickness, and over the rump region between the pin and hook bones to measure rump fat thickness at the distal terminal point of the superficial gluteus medius muscle. Images were
122 collected using the Blackbox Pro 5000 image capturing system (Biotronics Inc., Ames, IA). Prolactin concentrations were measured in steer serum as an indicator of toxicosis (Strickland et al., 1993) on the final day of grazing before fasting. Blood samples (7 mL) were collected by venipuncture from the jugular vein, allowed to clot at 3°C for 14 h, and centrifuged at 3,000 × g to separate the serum from whole cells. Serum samples were stored at −20°C, and the concentration of prolactin was estimated using radioimmunoassay (Bernard et al., 1993).
Pasture Responses Samples of above-ground forage were collected at approximately 2-wk intervals for nutritive-value analysis by collecting the forage from three 0.9-m2 quadrats (2.54-cm stubble height) within each pasture. The samples were collected to reflect what was available for grazing and not necessarily what was selectively grazed. These samples were collected on 12 May, 29 May, 16 June, 29 June, and 14 July in 2011, and on 23 April, 7 May, 21 May, 4 June, and 18 June in 2012. Whole vegetative tillers were also collected by clipping at the crown from approximately 50 randomly chosen plants in each pasture. Tillers were collected twice during each growing season (11 May and 17 June in 2011, and 20 April and 5 June in 2012) from pastures and separated into leaf blades and sheaths. All samples were stored at −20°C and lyophilized. After drying, samples were ground to 4 mm with a Wiley mill (Thomas Scientific, Swedesboro, NJ) and subsequently ground through a 1-mm screen using a cyclone mill. In vitro DM digestibility was determined using the filter-bag method described in Vogel et al. (1999) using rumen fluid collected from cannulated Holstein steers being fed a corn silage–alfalfa diet. Nitrogen content of the forage was determined via combustion with a Leco FP-215 N Analyzer (LECO Corp., St. Joseph, MI) and was converted to CP by multiplying by 6.25.
Goff et al.
Presence of endophyte-infected tall fescue was detected by measuring ergovaline concentrations using a HPLC with a florescence detection using modified procedures of Yates and Powell (1988) that were described by Carter et al. (2010). The percentages of tall fescue, orchardgrass, and other species within the pastures were estimated on 31 May 2011 and 28 May 2012 using a double-sampling technique (Ortega-Santos, 1990). Amounts of these species were visually estimated from 30 random 0.09-m2 quadrats (25 quadrats in 2012), and a curve was developed to correct for visual bias by harvesting and hand separating 3 quadrats per pasture to determine the actual percentage of each botanical fraction. Tall fescue seed-head densities were estimated by counting the number of reproductive culms within thirty 0.25-m2 quadrats on 16 June 2011 and 22 May 2012. Disk-meter-height recordings for monitoring grazing intensities were also used to estimate forage mass for subsequent determination of forage allowance (kg of DM/steer). Calibration equations were developed to convert measured compressed canopy heights into estimates of herbage mass (kg of DM/ha) by clipping herbage to the soil surface at 3 locations per pasture on 31 May 2011, 14 July 2011, 7 May 2012, and 18 June 2012. Forage samples were placed in a forced-air oven at 65°C for 2 d to correct for moisture content. It was necessary during 2012 to correct for dead plant material, which was primarily from dead warm-season species from the previous growing season. Corrections were done by estimating percentage of dead material using near infrared spectroscopy (NIRS). Therefore, samples for calibrating the DMH readings were ground to pass through a 1-mm screen with a cyclone mill (UDY Corp., Fort Collins, CO) and scanned with a Foss 6500M NIRS Instrument (Foss NIRSystems Inc., Laurel, MD). Samples collected for estimating botanical compositions were separated into green and dead material and
also ground scanned with the NIRS Instrument (Foss NIRSystems Inc.). A calibration for estimating dead material in the 2012 DMH samples was generated from the spectra of the known dead material percentages in botanical composition samples using the partial least squares approach (SE of calibration = 0.98; SE of crossvalidation = 5.04, SE of prediction = 3.93; R2 = 0.88). Separate linear calibration equations for estimating forage mass were created because of differences (P < 0.05) in slopes between years. Disk-meter calibrations were needed in 2011 for the different treatment combinations. The equations with herbicide treatment and low and moderate grazing intensities were kg of DM/ha = 54 + 414DMH (R2 = 0.94) and kg of DM/ ha = −224 + 548DMH (R2 = 0.90), respectively. Equations for without herbicide treatment and low and moderate grazing intensities were kg of DM/ha = 764 + 383DMH (R2 = 0.87) and kg of DM/ha = 1,391 +246DMH (R2 = 0.92), respectively. One regression accurately described the relationship between green forage mass and DMH in 2012 (kg of DM/ha = 495 + 191DMH; R2 = 0.80). Forage mass was used to calculate forage allowance (kg of DM/100 kg of BW). Body weights were those estimated for the mid-point of the grazing seasons in each year. These BW estimates were also used for calculating stocking rate on a BW basis (kg of BW/ha). Stocking rate for each pasture was calculated on a basis of steer numbers per hectare averaged over the grazing season (steer d/d on pasture) and on a total BW per acre basis.
Statistical Analysis Data were analyzed as a randomized complete block design with 3 replications using PROC MIXED in SAS (v.9.2, SAS Institute Inc., Cary, NC). Pastures were used as the experimental unit in all analyses. With and without seed-head suppression, grazing intensity, and their interaction were analyzed as fixed effects.
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Seed-head suppression of tall fescue
Table 1. Least squares means (SE of the mean1) for stocking rate (steers per hectare) and forage allowance (kg of DM/100 kg of BW) for combinations of with and without chemical seed-head suppression and low and moderate grazing intensities of steers grazing mixed pastures of tall fescue, orchardgrass, and other species
Response Stocking rate Steers/ha kg of BW/ha2,3 Forage allowance, kg of DM/100 kg of BW
With seed-head suppression
Without seed-head suppression
Grazing intensity
Grazing intensity
Low
Moderate
Low
Moderate
3.0c (0.01) 932 (35) 252a (27)
3.9b (0.3) 1,233 (110) 203c (42)
4.1b (0.3) 1,278 (93) 253a (44)
5.9a (0.2) 1,761 (113) 151c (24)
Least squares means within rows with different superscripts differ statistically, P < 0.05. Standard errors of the mean, shown in parentheses, were derived using the conservative calculation. 2 Interaction between seed-head suppression and grazing intensity was not significant (P = 0.177). 3 Body weights used in calculation of stocking rates and forage allowance were those estimated at the mid-points of the grazing seasons. a–c 1
Replanting done before the second grazing season negated evaluating year as a fixed effect; therefore, year and block were evaluated as random effects. Models analyzing nutritive values also evaluated sample collection number (5 sample dates in each year) within year and its interaction with the 2 treatments. Nutritive values were analyzed as repeated measures using the autoregression covariance structure (Littell et al., 2006). Nutritive-value data are presented in the results by date rate rather than sample collection number. Mean comparisons among treatments and their interactions were done using the PDIFF option of LSMEANS.
RESULTS AND DISCUSSION Treatment Targets Mean DMH averaged over the 2 grazing seasons for seed head–suppressed pastures with light and moderate grazing intensities were 7.2 ± 1.1 (forage mass = 2,263 ± 174 kg of DM/ha) and 6.1 ± 0.8 cm (forage mass = 2,276 ± 416 kg of DM/ha), respectively, and in nonsuppressed pastures were 8.7 ± 1.3 (forage mass = 3,081 ± 364 kg of DM/ha) and 7.3 ± 1.5 cm (2,539 ± 269 kg of DM/ ha), respectively. Overall, the targeted
DMH were met; however, mean DMH for the moderate grazing intensity of nonsuppressed pastures was slightly above the targeted range. Mean DMH for nonsuppressed pastures that were moderately grazed in the second grazing season was 7.6 ± 1.3 cm because of higher DMH during the early part of the season when there was considerable spot grazing that skewed the measures. Although herbage masses are numerically similar between the low and moderate grazing intensities for seed head–suppressed pastures, actual mean stocking rates and forage allowances with seed-head suppression differed (P < 0.05) between the 2 intensities (Table 1). Averaged over the 2 grazing seasons, there were fewer (P < 0.001) reproductive tillers in seed head–suppressed (21 ± 7 reproductive tillers/ m2) compared with nonsuppressed (90 ± 12 reproductive tillers/m2) pastures. Although Chaparral herbicide was effective in reducing seed heads, reproductive tiller densities were twice higher than the average of 10 reproductive tillers/m2 reported by Aiken et al. (2012) for endophyte-infected tall fescue treated with Chaparral. Ergovaline concentrations were zero for most of the whole tiller samples. Four of the 120 samples collected over both years had detectable ergova-
line concentrations, with the highest concentration in these 4 samples being 0.16 μg/g of DM. This provided strong evidence there was a small percentage of the fescue plants infected with the endophyte. Fribourg et al. (1991) concluded that signs of fescue toxicosis will not occur if infection percentages of tall fescue stands are less than 22%. Prolactin concentrations in steer serum did not differ (P = 0.398) between suppressed (241 ± 104 ng/ mL) and nonsuppressed (305 ± 102 ng/mL) pastures, which supports that only a small fraction of the plants were infected. Prolactin concentrations were highly variable and ranged in individual steers from 138 to 526 ng/mL. The greater densities of reproductive tiller in nonsuppressed pastures would likely have reduced prolactin concentrations in these pastures if a significant fraction of seed heads were infected and contained high concentrations of ergot alkaloids (Rottinghaus et al., 1991).
Pasture Productivity There was an interaction (P = 0.515) between seed-head suppression and grazing intensity on stocking rate (steers/ha; Table 1). Nonsuppressed pastures with the moderate
124 grazing intensity supported the most (P < 0.05) steers, whereas the lightly grazed, suppressed pastures supported the least (P < 0.05) number of steers. The lightly grazed, nonsuppressed and moderately grazed, suppressed pastures had similar (P = 0.515) stocking rates. However, there was no seedhead suppression × grazing intensity interaction (P = 0.177) with stocking rate on a BW basis. Greater BW was supported on nonsuppressed (1,519 ± kg of BW/ha) than suppressed (1,082 ± kg/ha) pastures and also was greater for moderately (1,497 ± kg of BW/ha) than lightly (1,105 ± kg of BW/ha) grazed pastures. Similar to Aiken et al. (2012), the tall fescue showed a lag in growth over the first few weeks after herbicide application. Although active growth was restored, the suppressed tall fescue appeared to remain behind in growth compared with nonsuppressed tall fescue. This resulted in nonsuppressed pastures supporting more steers, which was evident by the lightly grazed nonsuppressed pastures carrying a similar number of steers as moderately grazed suppressed pastures. Higher pasture stocking rates measured in experiments using put-andtake stocking are related to greater forage production (Mott, 1960). Wimer et al. (1986) reported that forage masses in smooth bromegrass pastures treated with mefluidide to
Goff et al.
suppress seed heads and grazed with cow-calf pairs (1.7 ha/cow-calf pair) averaged across the season approximately one-third of the forage produced by the control pastures. Aiken et al. (2012) reported less forage for seed head–suppressed pastures during the first year of the study but showed that warmer temperatures in the spring resulted in comparable levels of forage between herbicide treatments during the spring of the second year. Results of Aiken et al. (2012) and the present experiment strongly indicate seed head–suppressed tall fescue pastures will not support as many animals as nonsuppressed pastures. As mentioned previously, there were concerns of overgrazing in the seed head–suppressed pastures of endophyte-free fescue during the first growing season.
Botanical Composition Percentage tall fescue was less (P < 0.05) in suppressed (62.1 ± 2.3%) than nonsuppressed (69.7 ± 1.9%) pastures, and tended (P = 0.057) to contribute less DM in moderately (63.1 ± 2.2%) than lightly (68.9 ± 2.1%) grazed pastures (Figure 1). Orchardgrass DM percentage was not affected (P > 0.4) by seed-head suppression or grazing intensity. Percentage of the other species component was greater in suppressed (23.1 ±
Figure 1. Percentages and SE bars for endophyte-free tall fescue in the canopy DM in pasture mixture with orchardgrass and other grasses that were with or without chemical seed-head suppression and grazed with light or moderate grazing intensities.
3.1%) than nonsuppressed (15.9 ± 2.6%) pastures but was not affected (P = 0.382) by grazing intensity. This fraction was composed primarily of Kentucky bluegrass (Poa pratensis L.), crabgrass (Digitaria spp.), common chickweed [Stellaria media (L.) Vill.], purple deadnettle (Lamium purpureum L.), henbit (Lamium amplexicaule L.), and buckhorn plantain (Plantago coronopus L.) in 2011 and Kentucky bluegrass, crabgrass, and buckhorn plantain in 2012. Because the botanical composition of the pastures was determined on a DM basis, it was not surprising that Chaparral would lower the tall fescue biomass because of the inhibition of reproductive growth and its corresponding loss in DM. Chaparral herbicide does not affect reproductive development or growth of orchardgrass or Kentucky bluegrass (Aiken et al., 2012). Aiken et al. (2012) similarly reported a reduction in tall fescue of approximately 15% in Chaparral treatments within a tall fescue–Kentucky bluegrass mixture. Cattle also have been visually observed to selectively graze seed head–suppressed tall fescue (Aiken et al., 2012). This was indicated by the trend of reduced DM percentages of fescue in moderately grazed pastures.
Nutritive Values CP. There was an interaction (P < 0.05) between seed-head suppression and sample collection number in both years. Crude protein percentages in available forage was greater (P < 0.001) with suppression than without across all sampling dates in each year (Figure 2). Turner et al. (1990a) reported that N% in grazed endophyte-infected tall fescue that was treated with mefluidide to suppress seed heads was higher than in nonsuppressed fescue for samples collected from May to early July but not for those collected from late June to September. In the 2012 grazing season, CP in suppressed pastures was stable (P > 0.25) across sample dates, whereas it declined (P < 0.01) in nonsuppressed pastures between the
Seed-head suppression of tall fescue
Figure 2. Means and SE bars for sampling dates in 2011 (A) and 2012 (B) for CP percentages in available forage of pastures of endophyte-free tall fescue in mixture with orchardgrass and other grasses that were with or without chemical seed-head suppression. Asterisks above sampling dates denote differences (P < 0.05) between suppressed or unsuppressed pastures. Different letters between sampling dates for a given trend line are significantly different (P < 0.05).
23 April and 7 May sampling dates in the 2012 season. Crude protein in 2011 declined (P < 0.05) in suppressed and nonsuppressed pasture between the 29 May and 16 June sampling dates and stabilized for the remainder of the season.
Leaf blade and sheath components of suppressed vegetative tillers of fescue had greater percentages of CP than those components in nonsuppressed vegetative tillers across both years and sampling dates (Table 2). Leaf blades sampled 11 May in 2011
125 and leaf sheaths on 17 June in 2012 were similar in CP between suppressed vegetative tillers and nonsuppressed reproductive tillers, but CP was greater in suppressed blades and sheaths for the other dates. With the low density of reproductive tillers in suppressed pastures, protein and nonprotein N in vegetative tillers of suppressed pastures may have been conserved rather than being translocated to reproductive tissues. There was also an interaction (P < 0.05) between seed-head suppression and grazing intensity on CP of the available forage that was consistent (P = 0.104) across both years. Crude protein in suppressed pastures was not affected (P = 0.498) by grazing intensity, but plants in nonsuppressed pastures showed higher CP content (P < 0.001) in lightly (10.4%) than moderately (9.5%) grazed pastures. Low-quality stems (Collins and Fritz, 2003) likely had a greater presence in the nonsuppressed pastures under the light grazing intensity because of reduced grazing of plant growing points in the early season. Conversely, lack of difference between grazing intensities with seed-head suppression reflects a low density of stems through inhibition of stem elongation from the herbicide treatment. The difference in CP between seedhead suppression treatments was approximately 4 percentage units for the forage samples collected at each date. Increases in CP with suppression can be attributed to inhibition of tall fescue reproductive growth that will reduce the presence of stems and conserve nonprotein nitrogen in vegetative tissues that would normally be translocated to stem and seed tissues during reproductive development. Stems are inherently low in CP, whereas young vegetative tissues contain high levels of CP (Collins and Fritz, 2003). An increase in CP is common for most studies that have focused on the chemical suppression of reproductive growth (Wimer et al., 1986; Roberts and Moore, 1990; Moyer and Kelley, 1995). Aiken et al. (2012) reported higher forage CP concentrations in Chaparral-treated
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Table 2. Percentages of CP and IVDMD in the DM of leaf blades and sheath tissues of tall fescue in each year and sampling date that were separated from seed head–suppressed vegetative (Veg.) and nonsuppressed vegetative and reproductive (Rep.) tillers Seed-head suppression
Item
Leaf component
Year
Sample date
CP, % IVDMD, %
Blade Sheath Leaf Sheath
2011 2012 2011 2012 2011 2012 2011 2012
11 May 17 June 20 Apr 5 June 11 May 17 June 20 Apr 5 June 11 May 17 June 20 Apr 5 June 11 May 17 June 20 Apr 5 June
With
Without
Veg.
Veg.
Rep.
SEM1
18.9a 15.6a 16.2a 15.3a 11.2a 7.9a 11.1a 8.6a 83.7 75.4a 75.3 74.6a 81.0a 80.6a 72.3b 78.3a
15.9b 13.4b 11.1c 12.9b 8.7b 6.7b 6.8b 7.0b 83.7 72.3ab 71.2 71.2a 79.8a 79.7a 75.8a 77.3a
18.7a 11.1c 14.4b 8.0c 8.5b 8.0a 6.4b 6.1b 81.5 69.2b 75.1 57.2b 63.9b 56.4b 59.7c 52.4b
0.8 0.5 0.7 0.2 0.6 0.7 0.6 0.7 0.7 1.0 2.2 2.9 1.1 1.9 1.9 0.7
Means within leaf-component, year, and sampling-date groups with different superscripts differ statistically, P < 0.05. Standard error of the means were derived using the conservative calculation.
a–c 1
pastures, especially after the forage in the control pastures had entered the reproductive stages of development. IVDMD. Grazing intensity did not affect (P = 0.115) IVDMD of available forage, but there was a seed-head suppression × sample collection number interaction on IVDMD in both years. In vitro DM disappearance of available forage in 2011 was greater (P < 0.001) in suppressed than in nonsuppressed pastures across all sampling dates (Figure 3); however, a difference between suppressed and nonsuppressed pastures was detected (P < 0.05) in 2012 for only the last 3 sample dates. In support of these results, Turner et al. (1990a) reported greater in vitro OM digestibility of grazed endophyte-infected tall fescue treated with mefluidide than without the treatment from May to early August. Across sampling dates in 2011, IVDMD of available forage in suppressed pastures declined (P < 0.05) between the first and third sampling dates and was stable (P > 0.440) between the third and fifth dates,
whereas IVDMD for suppressed pastures in 2012 stabilized (P > 0.560) between the second and fifth sampling dates. For nonsuppressed pastures, IVDMD declined (P < 0.001) over the first 3 sample dates and then stabilized (P > 0.210) between the third and fifth sample dates. Leaf blades and sheaths of vegetative tillers in suppressed and nonsuppressed pastures did not differ (P > 0.12) in IVDMD for all sample dates in both years except for sheaths on 20 April in 2012 when IVDMD was greater for nonsuppressed than suppressed tillers. The components of vegetative tillers had greater (P < 0.05) IVDMD than reproductive tillers for all dates other than the first sampling dates for leaf blades when there were no differences between vegetative and reproductive tillers. The increases in IVDMD of available forage with seed-head suppression was therefore indicated due to a reduced presence of reproductive tillers that dampens the digestibility of the available forage. This dampening of
the IVDMD of reproductive tillers appeared to be from maturation of leaf blades during the season combined with maturation and possible movement of carbohydrates from sheath to stem and seed tissues during reproductive development. Aiken et al. (2012) reported that IVDMD of forage was similar between control and seed head–suppressed tall fescue pastures and that forage IVDMD decreased in the control pastures at the onset of reproductive growth. Lower IVDMD of forage in nonsuppressed pastures could be attributed to more reproductive growth. The concentrations of structural carbohydrates, such as cellulose and hemicelluloses, are greater in stems compared with the leaves and sheaths of vegetative tissues and are less degradable within the digestive system of the ruminants (Moore and Hatfield, 1994). Digestibility of reproductive tissues is further reduced by the continual deposition of these carbohydrates and lignin with the cell wall as the plant matures. Lignin, a phenolic conglom-
Seed-head suppression of tall fescue
127
erate bound to hemicelluloses, reduces digestibility by impeding access to the carbohydrates by the degrading microbes (Jung and Deetz, 1993).
Steer Performance Steer ADG was greater (P < 0.05) with seed-head suppression than without (Table 3), and with no interaction between suppression and grazing intensity (P = 0.619). Higher steer BW gains on pastures of endophyte-infected tall fescue treated with mefluidide for seed-head suppression than those untreated was also reported by Turner et al. (1990b). The ADG response could be attributed to the higher CP and IVDMD of available forage in suppressed pastures (Table 2). The ultrasound-detected carcass-related traits reflected the higher weight-gain efficiency on seed head–suppressed pastures. Longissimus dorsi areas and subcutaneous fat over the rump and between the 12th and 13th ribs were greater (P < 0.05) in steers grazing suppressed pastures, which corresponded with visual observations of body size and condition. Aiken et al. (2002) concluded that ultrasonic measures of longissimus dorsi and subcutaneous fat are to be expected with positive growth rate responses to grazing treatments. The increased gain with seed-head suppression did not result in greater total BW gain per hectare, which tended to be higher (P < 0.068) without than with suppression. This was because stocking rates tended to be higher for nonsuppressed (5.0 ± 0.2 steers/ha) than suppressed (3.4 ± 0.2 steers/ha) pastures (Table 1).There was a strong tendency (P = 0.057) of an interaction between seed-head suppression and grazing intensity on average stocking rates (steers/ha) over the grazing seasons, but this was because stocking rates did not differ (P = 0.52) between the low grazing intensity with suppression (4.1 ± 0.3 steers/ha) and the moderate grazing intensity without seed-head suppression (4.1 ± 0.2 steers/ha). Stocking rates based on BW, however, were
Figure 3. Means and SE bars for sampling dates in 2011 (A) and 2012 (B) for IVDMD of available forage in pastures of endophyte-free tall fescue in mixture with orchardgrass and other grasses that were treated with or without chemical seed-head suppression. Asterisks above sampling dates denote differences (P < 0.05) between suppressed or unsuppressed pastures. Different letters between sampling dates for a given trend line are significantly different (P < 0.05).
greater for nonsuppressed (1,519 ± 75 kg of BW/ha) than suppressed (1,082 ± 110 kg of BW/ha), and there was a seed-head suppression × grazing intensity interaction (P = 0.177). Higher ADG for seed head–suppressed pastures apparently was not enough
to compensate for the lower numbers of steers that these pastures supported. Higher stocking rates with put-andtake experiments reflect greater forage production (Mott, 1960). Wimer et al. (1986) reported that forage
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Table 3. Mean weight gains and ultrasound-detected carcass-related traits for seed-head suppression of tall fescue and grazing intensity treatments for steers grazing mixed pastures of tall fescue, orchardgrass, and other forages Seed-head suppression Item1 ADG, kg/d BWG/ha, kg/ha LDA, cm2 Backfat, mm Rump fat, mm
With 0.94 (0.06) 253d (26) 56.3c (1.9) 3.6a (0.2) 3.2a (0.2) a
2
Grazing intensity
Without
Low
Moderate
0.79 (0.09) 304c (40) 50.9d (2.1) 3.2b (0.2) 2.9b (0.2)
0.93 (0.06) 251b (22) 54.0 (2.2) 3.4 (0.2) 3.1 (0.2)
0.80b (0.09) 305a (40) 53.1 (2.3) 3.3 (0.2) 3.0 (0.2)
b
a
Least squares means within treatment groups with different superscripts differ statistically, P < 0.05. Least squares means within treatment groups with different superscripts differ statistically, P < 0.10. 1 BWG/ha = BW gain per hectare; LDA = longissimus dorsi area. 2 Standard errors of the mean, shown in parentheses, were derived using the conservative calculation. a,b c,d
masses in smooth bromegrass pastures treated with mefluidide to suppress seed heads and grazed with cow-calf pairs (1.7 ha/cow-calf pair) averaged across the season approximately onethird of the forage produced by the control pastures. Aiken et al. (2012) reported less forage for seed head– suppressed pastures during the first year of the study but showed that warmer temperatures in the spring resulted in comparable levels of forage between herbicide treatments during the spring of the second year. Results of Aiken et al. (2012) and the present experiment strongly indicate seed head–suppressed tall-fescue pastures will not support as many animals as unsuppressed pastures. As mentioned previously, there were concerns of overgrazing in the seed head–suppressed pastures of endophyte-free fescue during the first growing season. Furthermore, Turner et al. (1990b) reported steers grazing endophyteinfected tall fescue treated with mefluidide to have 1.5-fold higher estimated OM intake than those grazing untreated tall fescue. Average daily gains were higher (P < 0.05) for steers on the low grazing intensity (3.5 ± steers/ha) treatment compared with the moderate grazing intensity (4.9 ± steers/ha). Average stocking rates on a BW basis for light and moderate grazing intensities were 515 ± 32 and 698 ± 49 kg of BW/
ha, respectively. This was a typical response for stocking rate studies (Mott, 1960; Jones and Sandland, 1974). Animals are able to selectively graze plant tissues that maximize the nutritive value (i.e., leaf tissue) of the forage under low grazing pressures, which results in a higher quality diet and better animal performance (Blaser et al., 1956; Mott; 1960; Bryant et al., 1965). Increases in stocking pressures create greater competition for available forage. Animals are then forced to be less selective in their diet and will consume plant tissues that are of lower nutritive value (i.e., stem), lowering animal performance (Blaser et al., 1956; Mott; 1960; Bryant et al., 1965).
IMPLICATIONS Chemical seed-head suppression improved forage nutritive value and increased steer ADG on pastures that contain 60 to 70% endophyte-free tall fescue in mixture with orchardgrass and other grasses. The ADG with seed-head suppression was 19% greater than without suppression. This was less than the 39% increase reported by Aiken et al. (2012) for steers grazing seed head–suppressed endophyteinfected tall fescue. This suggests that enhancement of forage quality through suppression of lower-quality reproductive tissues will benefit calf
performance, but the alleviation of toxic seed heads from endophyte-infected tall fescue could have a greater influence on steer performance.
LITERATURE CITED Aiken, G. E., F. M. Rouquette Jr., and R. K. Miller. 2002. Grazing treatment effects on ultrasound measures of 12th to 13th rib fat thickness and longissimus area. Prof. Anim. Sci. 18:368–372. Aiken, G. E., B. M. Goff, W. W. Witt, B. B. Sleugh, and P. L. Burch. 2012. Steer and plant responses to chemical suppression of seedhead emergence in toxic endophyte-infected tall fescue. Crop Sci. 52:960–969. Bacon, C. W., J. K. Porter, J. D. Robbins, and E. S. Luttrell. 1977. Epichloë typhina from toxic tall fescue grasses. Appl. Environ. Microbiol. 34:576–581. Bernard, J. K., A. B. Chestnut, B. H. Erickson, and F. M. Kelly. 1993. Effects of prepartum consumption of endophyte-infested tall fescue on serum prolactin and subsequent milk production of Holstein cow. J. Dairy Sci. 76:1928–1933. Blaser, R. E., R. C. Hammes Jr., H. T. Bryant, C. M. Kincaid, W. H. Skrdla, T. H. Taylor, and W. L. Griffith. 1956. The value of forage species and mixtures for fattening of steers. Agron. J. 48:508–513. Bransby, D. I., A. G. Matches, and G. F. Krause. 1977. Disk meter for rapid estimation of herbage yield in grazing trails. Agron. J. 69:393–396. Bryant, H. T., R. C. Hammes Jr., R. E. Blaser, and J. P. Fontenot. 1965. Effect of stocking pressure on animal and acre output. Agron. J. 57:573–576.
Seed-head suppression of tall fescue Carter, J. M., G. E. Aiken, C. T. Dougherty, and F. N. Schrick. 2010. Steer responses to feeding soybean hulls and steroid hormone implantation on toxic tall fescue pasture. J. Anim. Sci. 88:3759–3766. Collins, M., and J. O. Fritz. 2003. Forage quality. Pages 363–390 in Forage. Vol. I: An Introduction to Grassland Agriculture. 6th ed. R. F. Barnes, C. J. Nelson, M. Collins, and K. J. Moore, ed. Blackwell Publ., Ames, IA. Fribourg, H. A., A. B. Chestnut, R. W. Thompson, J. B. McLaren, R. J. Carlisle, K. D. Gwinn, M. C. Dixon, and M. C. Smith. 1991. Steer performance in fescue-clover pastures with different levels of endophyte infestation. Agron. J. 83:777–781. Goff, B. M., G. E. Aiken, W. W. Witt, B. B. Sleugh, and P. L. Burch. 2012. Steer consumption and ergovaline recovery from in vitro digested residues of tall fescue seedheads. Crop Sci. 52:1437–1440. Hoveland, C. S., R. L. Haaland, C. C. King Jr., W. B. Anthony, E. M. Clark, J. A. McGuire, L. A. Smith, H. W. Grimes, and J. L. Holliman. 1980. Association of Epichloe typhina fungus and steer performance on tall fescue pasture. Agron. J. 72:1064–1065. Jones, R. J., and R. L. Sandland. 1974. The relation between animal gain and stocking rate: Derivation of the relation from the results of grazing trials. J. Agric. Sci. 83:335–342. Jung, H. G., and D. A. Deetz. 1993. Cell wall lignification and degradability. Pages 315–346 in Forage Cell Wall Structure and Digestibility. H. G. Jung, D. R. Buxton, R. D. Hatfield, and J. Ralph ed. Am. Soc. Agron., Crop Sci. Soc. Am., Soil Sci. Soc. Am., Madison, WI.
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Littell, R. C., G. A. Milliken, W. W. Stroup, R. D. Wolfinger, and O. Schabenberger. 2006. SAS for Mixed Models. 2nd ed. SAS Institute Inc., Cary NC.
Sheaffer, C. C., and G. C. Marten. 1986. Effect of mefluidide on cool-season perennial grass forage yield and quality. Agron. J. 78:75–79.
Moore, K. J., and R. D. Hatfield. 1994. Carbohydrates and forage quality. Pages 229–280 in Forage Quality, Evaluation, and Utilization. G. C. Fahey Jr., M. Collins, and D. R. Mertens, ed. Soc. Agron., Crop Sci. Soc. Am., Soil Sci. Soc. Am., Madison, WI.
Strickland, J. R., J. W. Oliver, and D. L. Cross. 1993. Fescue toxicosis and its impact on animal agriculture. Vet. Hum. Toxicol. 35:454–464.
Mott, G. O. 1960. Grazing pressure and the measurement of pasture production. Pages 606–611 in Proc. VIII Int. Grass. Congr. Alden Press, Oxford, UK. Moyer, J. L., and K. W. Kelley. 1995. Broadleaf herbicide effects on tall fescue (Festuca arundinacea) seedhead density, forage yield, and quality. Weed Technol. 9:270–276. Ortega-Santos, J. A. 1990. Ecophysiological responses of rhizoma peanut to grazing management. PhD Diss. Univ. Florida, Gainesville. Roberts, C. A., and K. J. Moore. 1990. Chemical regulation of tall fescue reproductive development and quality with amidochlor. Agron. J. 82:523–526. Rottinghaus, G. E., G. B. Garner, C. N. Cornell, and J. L. Ellis. 1991. HPLC method for quantitating ergovaline in endophyte-infected tall fescue: Seasonal variation of ergovaline levels in stems with leaf sheaths, leaf blades, and seed heads. J. Agric. Food Chem. 39:112–115. Schmidt, S. P., and T. G. Osborn. 1993. Effects of endophyte-infected tall fescue on animal performance. Agric. Ecosyst. Environ. 44:233–262.
Thompson, F. N., J. A. Stuedemann, and N. S. Hill. 2001. Anti-quality factors associated with alkaloids in eastern temperate pasture. J. Range Manage. 54:474–489. Turner, K. E., J. A. Paterson, J. S. Kerley, and J. R. Forwood. 1990a. Mefluidide treatment of tall fescue pastures: Forage quality. J. Anim. Sci. 68:3406–3411. Turner, K. E., J. A. Paterson, J. S. Kerley, and J. R. Forwood. 1990b. Mefluidide treatment of tall fescue pastures: Intake and animal performance. J. Anim. Sci. 68:3399– 3405. Vogel, K. P., J. F. Pedersen, S. D. Masterson, and J. J. Toy. 1999. Evaluation of filter bag system for NDF, ADF, and IVDMD analysis. Crop Sci. 39:276–279. Wimer, S. K., J. K. Ward, B. E. Anderson, and S. S. Waller. 1986. Mefluidide effects on smooth bromegrass composition and grazing cow-calf performance. J. Anim. Sci. 63:1054–1062. Yates, S. G., and R. G. Powell. 1988. Analysis of ergopeptine alkaloids in endophyte infected tall fescue. J. Agric. Food Chem. 36:337–340.
Forage nutritive value and steer responses to grazing intensity and seed-head suppression of endophyte-free tall fescue in mixed pastures1
B. M. Goff,* G. E. Aiken,†2 PAS, W. W. Witt,* P. L. Burch,‡ and F. N. Schrick§ *Department of Plant and Soil Sciences, University of Kentucky, Lexington 40546; †USDA-ARS, Forage and Animal Production Research Unit, Lexington, KY 40546; ‡P. L. Dow AgroSciences, Christiansburg, VA 24073; and §Department of Animal Sciences, University of Tennessee, Knoxville 37996-4574
ABSTRACT A 2-yr grazing experiment was conducted with 8- to 10-mo old steers on pastures of endophyte-free tall fescue [Lolium arundinaceum (Schreb.) Darbysh.] in mixture with other grasses to assess the effect of seed-head suppression of fescue on steer performance and forage nutritive values. Treatments with and without seed-head suppression were each combined with either light or moderate grazing intensities for assignment to twelve 1.0-ha pastures of the grass mixtures. The experiment was conducted as a randomized complete block design with 3 replications. Steer ADG was measured, and CP and in vitro DM digest-
1 Mention of trade names or commercial products in the article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA. The USDA is an equal opportunity employer. 2 Corresponding author: glen.aiken@ars. usda.gov
ibility (IVDMD) of available forage, and leaf blade and sheaths of fescue tillers, were monitored. Averaged over grazing intensities, ADG was 16% greater (P < 0.05) with seed-head suppression; however, a lower (P < 0.001) mean stocking rate with seed-head suppression resulted in a tendency of greater (P = 0.068) BW gain per hectare without suppression. Crude protein of available forage was consistently greater (P < 0.01) with seed-head suppression across all dates, whereas IVDMD was consistently greater (P < 0.01) with seed-head suppression in the late grazing season. Crude protein in leaf blades and sheaths of vegetative tillers with seed-head suppression were consistently greater (P < 0.01) than vegetative tillers without seed-head suppression. The IVDMD of blades and sheaths was similar (P > 0.18) between suppressed and nonsuppressed vegetative tillers, and both had greater (P < 0.05) IVDMD than reproductive tillers over most dates. Results showed seed-head suppression of fescue to improve steer ADG by increasing CP in vegetative tissues and improving digestibility of avail-
able forage by alleviating lower-quality, reproductive tillers. Key words: beef cattle, forage quality, plant growth regulation, seed-head suppression, tall fescue
INTRODUCTION Tall fescue [Lolium arundinaceum (Schreb.) Darbysh.] is a cool-season perennial grass that is used as a forage on approximately 14 million ha in the humid east (Thompson et al., 2001). Tall fescue is persistent and productive, but cattle production and thriftiness is low (Hoveland et al., 1980) because of toxic ergot alkaloids produced by a fungal endophyte (Neotyphodium coenophialum) that infects most plants of tall fescue (Bacon et al., 1977). Cattle that consume endophyte-infected tall fescue can undergo a toxicosis that causes cattle to exhibit poor weight gain, rough hair coats during the summer months, elevated core body temperatures, and reduced serum prolactin concentra-
Seed-head suppression of tall fescue
tions (Schmidt and Osborn, 1993; Strickland et al., 1993). Removal of fescue seed heads can be an effective strategy for reducing dosage of alkaloids by grazing animals because ergot alkaloid concentrations are greater in seeds than other plant parts (Rottinghaus et al., 1991) and cattle readily consume seed heads (Goff et al., 2012). Seed heads can be removed by mowing, but this is generally ineffective if done during seed maturation because cattle selectively graze seed heads over a short period during early seed development (Goff et al., 2012). Research demonstrated that the plant growth regulator mefluidide inhibits floral development of cool-season perennial grasses (Roberts and Moore, 1990; Moyer and Kelley, 1995) to maintain vegetative stands and improve nutritive value (Sheaffer and Marten, 1986), but mefluidide was not licensed for forages. Aiken et al. (2012) reported that steer ADG increased and signs of fescue toxicosis were mitigated on chemically seed head–suppressed tall fescue that was grazed; however, it could not be concluded whether the increase in weight gain was due to alleviation of toxic seed heads or by enhancement of nutritive value by maintaining fescue in the vegetative stage of growth. A follow-up grazing experiment was designed to use endophyte-infected fescue to evaluate the interaction between seed-head suppression and grazing intensity on steer performance and pasture responses. However, these pastures were inadvertently planted with endophyte-free seed. Nonetheless, this provided pastures of endophyte-free fescue that removed the ergot alkaloid effect and provided an objective evaluation of steer and forage nutritive-value responses to chemical seed-head suppression and light and moderate grazing intensities.
MATERIALS AND METHODS Experimental Site and Design The study was conducted at the University of Kentucky C. Oran Little Research Center, near Versailles, Ken-
tucky, on a McAfee silt loam (fine, mixed, active, mesic Mollic Hapludalf) and a Maury-Bluegrass silt loam (fine, mixed, active mixed, mesic Typic Paleudalf) complex. Twelve 1.0-ha pastures were sprayed twice with glyphosate (5 L/ha) and no-till planted with endophyte-free Kentucky 31 tall fescue on 19 March 2010 with 28 kg of pure live seed per hectare. Areas that exhibited poor emergence or losses of plants from intensive spot grazing were reseeded on 14 September 2011 at the same rate. Pastures were blocked according to the average slope and soil type. Two grazing intensities (light and moderate) and 2 rates of herbicide application were arranged as a 2 × 2 factorial with 3 replications. Herbicide treatments consisted of Chaparral {Dow AgroSciences, Indianapolis, IN; 62.13% aminopyralid [4-amino-3,6-dichloro-2-pyridinecarboxylic acid] and 9.45% metsulfuron-methyl [methyl 2-[[[[(4-methoxy-6-methyl-1,3,5triazin-2-yl)amino]carbonyl]amino] sulfonyl]benzoate]} applied at 0 or 140 g/ha (metsulfuron: 13.2 g/ha, aminopyralid: 87.0 g/ha) on 7 April 2011 and 19 March 2012. Control pastures received Milestone (DowAgrosciences LLC, Indianapolis, IN; 40.6% aminopyralid) at 220 g/ha (aminopyralid: 89.3 g/ha) on the same dates. All herbicides were applied with nonionic surfactant (0.25% vol/vol). Pastures were fertilized annually in mid-March with 78 kg of N/ha.
Steer Responses Steers used in 2010 were primarily crossbred Angus with initial BW of 263 ± 19 kg, and those used in 2011 were predominately Hereford, Charolais, and Angus with initial BW of 314 ± 11 kg in 2011. Tester steers were blocked by BW in 2010 and by BW and breed type in 2011 for assignment to pastures such that mean BW were similar across pastures and breed types. On the day that grazing was initiated, all steers were dewormed using moxidectin (Cydectin; Fort Dodge Animal Health, Fort Dodge, IA) and received steroidal
121 implants (Synovex; 200 mg of progesterone, 20 mg of estradiol; Fort Dodge Animal Health). Steers on pasture were provided free-choice trace minerals (zinc, 0.35% minimum; manganese, 0.2% minimum; iron, 0.2% minimum; copper, 0.03% minimum; selenium, 0.009% maximum; iodine, 0.007% minimum; cobalt, 0.005% minimum). Steers were maintained according to University of Kentucky Institutional Animal Care and Use Committee (IACUC) protocol #2011– 0797. Steers grazed the pastures for 74 d (5 May to 14 July) and 84 d (3 April to 26 June) for the 2011 and 2012 growing seasons, respectively. Three tester steers were used in each pasture, and put-and-take steers were used to provide light and moderate grazing intensities. Grazing intensities were established and monitored using a disk meter, similar in design to one described by Bransby et al. (1977), with the exception that the falling plate weighed 1.9 kg and had a diameter of 45 cm. Based on previous research (Aiken et al., 2012) and experience measuring forage mass with disk meters for seed head–suppressed and nonsuppressed pastures, mean disk meter heights (DMH) of 7 to 9 cm were targeted for the light grazing and 5 to 7 cm for the moderate intensity. Fifty DMH were randomly measured for each pasture at approximately 2-wk intervals. Stocking adjustments were made following the 2-wk measurements of DMH. Steers were weighed following a 12to 14-h fast at the start and end of each grazing season. Carcass-related traits of the tester steers were determined at the end of the grazing season using an Aloka SSD-500V (Hitachi Aloka Medical Ltd., Tokyo, Japan) ultrasound with a 3.5-MHz linear array transducer (UST 6049). Scans were taken between the 12th and 13th ribs to determine the longissimus dorsi area and backfat thickness, and over the rump region between the pin and hook bones to measure rump fat thickness at the distal terminal point of the superficial gluteus medius muscle. Images were
122 collected using the Blackbox Pro 5000 image capturing system (Biotronics Inc., Ames, IA). Prolactin concentrations were measured in steer serum as an indicator of toxicosis (Strickland et al., 1993) on the final day of grazing before fasting. Blood samples (7 mL) were collected by venipuncture from the jugular vein, allowed to clot at 3°C for 14 h, and centrifuged at 3,000 × g to separate the serum from whole cells. Serum samples were stored at −20°C, and the concentration of prolactin was estimated using radioimmunoassay (Bernard et al., 1993).
Pasture Responses Samples of above-ground forage were collected at approximately 2-wk intervals for nutritive-value analysis by collecting the forage from three 0.9-m2 quadrats (2.54-cm stubble height) within each pasture. The samples were collected to reflect what was available for grazing and not necessarily what was selectively grazed. These samples were collected on 12 May, 29 May, 16 June, 29 June, and 14 July in 2011, and on 23 April, 7 May, 21 May, 4 June, and 18 June in 2012. Whole vegetative tillers were also collected by clipping at the crown from approximately 50 randomly chosen plants in each pasture. Tillers were collected twice during each growing season (11 May and 17 June in 2011, and 20 April and 5 June in 2012) from pastures and separated into leaf blades and sheaths. All samples were stored at −20°C and lyophilized. After drying, samples were ground to 4 mm with a Wiley mill (Thomas Scientific, Swedesboro, NJ) and subsequently ground through a 1-mm screen using a cyclone mill. In vitro DM digestibility was determined using the filter-bag method described in Vogel et al. (1999) using rumen fluid collected from cannulated Holstein steers being fed a corn silage–alfalfa diet. Nitrogen content of the forage was determined via combustion with a Leco FP-215 N Analyzer (LECO Corp., St. Joseph, MI) and was converted to CP by multiplying by 6.25.
Goff et al.
Presence of endophyte-infected tall fescue was detected by measuring ergovaline concentrations using a HPLC with a florescence detection using modified procedures of Yates and Powell (1988) that were described by Carter et al. (2010). The percentages of tall fescue, orchardgrass, and other species within the pastures were estimated on 31 May 2011 and 28 May 2012 using a double-sampling technique (Ortega-Santos, 1990). Amounts of these species were visually estimated from 30 random 0.09-m2 quadrats (25 quadrats in 2012), and a curve was developed to correct for visual bias by harvesting and hand separating 3 quadrats per pasture to determine the actual percentage of each botanical fraction. Tall fescue seed-head densities were estimated by counting the number of reproductive culms within thirty 0.25-m2 quadrats on 16 June 2011 and 22 May 2012. Disk-meter-height recordings for monitoring grazing intensities were also used to estimate forage mass for subsequent determination of forage allowance (kg of DM/steer). Calibration equations were developed to convert measured compressed canopy heights into estimates of herbage mass (kg of DM/ha) by clipping herbage to the soil surface at 3 locations per pasture on 31 May 2011, 14 July 2011, 7 May 2012, and 18 June 2012. Forage samples were placed in a forced-air oven at 65°C for 2 d to correct for moisture content. It was necessary during 2012 to correct for dead plant material, which was primarily from dead warm-season species from the previous growing season. Corrections were done by estimating percentage of dead material using near infrared spectroscopy (NIRS). Therefore, samples for calibrating the DMH readings were ground to pass through a 1-mm screen with a cyclone mill (UDY Corp., Fort Collins, CO) and scanned with a Foss 6500M NIRS Instrument (Foss NIRSystems Inc., Laurel, MD). Samples collected for estimating botanical compositions were separated into green and dead material and
also ground scanned with the NIRS Instrument (Foss NIRSystems Inc.). A calibration for estimating dead material in the 2012 DMH samples was generated from the spectra of the known dead material percentages in botanical composition samples using the partial least squares approach (SE of calibration = 0.98; SE of crossvalidation = 5.04, SE of prediction = 3.93; R2 = 0.88). Separate linear calibration equations for estimating forage mass were created because of differences (P < 0.05) in slopes between years. Disk-meter calibrations were needed in 2011 for the different treatment combinations. The equations with herbicide treatment and low and moderate grazing intensities were kg of DM/ha = 54 + 414DMH (R2 = 0.94) and kg of DM/ ha = −224 + 548DMH (R2 = 0.90), respectively. Equations for without herbicide treatment and low and moderate grazing intensities were kg of DM/ha = 764 + 383DMH (R2 = 0.87) and kg of DM/ha = 1,391 +246DMH (R2 = 0.92), respectively. One regression accurately described the relationship between green forage mass and DMH in 2012 (kg of DM/ha = 495 + 191DMH; R2 = 0.80). Forage mass was used to calculate forage allowance (kg of DM/100 kg of BW). Body weights were those estimated for the mid-point of the grazing seasons in each year. These BW estimates were also used for calculating stocking rate on a BW basis (kg of BW/ha). Stocking rate for each pasture was calculated on a basis of steer numbers per hectare averaged over the grazing season (steer d/d on pasture) and on a total BW per acre basis.
Statistical Analysis Data were analyzed as a randomized complete block design with 3 replications using PROC MIXED in SAS (v.9.2, SAS Institute Inc., Cary, NC). Pastures were used as the experimental unit in all analyses. With and without seed-head suppression, grazing intensity, and their interaction were analyzed as fixed effects.
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Table 1. Least squares means (SE of the mean1) for stocking rate (steers per hectare) and forage allowance (kg of DM/100 kg of BW) for combinations of with and without chemical seed-head suppression and low and moderate grazing intensities of steers grazing mixed pastures of tall fescue, orchardgrass, and other species
Response Stocking rate Steers/ha kg of BW/ha2,3 Forage allowance, kg of DM/100 kg of BW
With seed-head suppression
Without seed-head suppression
Grazing intensity
Grazing intensity
Low
Moderate
Low
Moderate
3.0c (0.01) 932 (35) 252a (27)
3.9b (0.3) 1,233 (110) 203c (42)
4.1b (0.3) 1,278 (93) 253a (44)
5.9a (0.2) 1,761 (113) 151c (24)
Least squares means within rows with different superscripts differ statistically, P < 0.05. Standard errors of the mean, shown in parentheses, were derived using the conservative calculation. 2 Interaction between seed-head suppression and grazing intensity was not significant (P = 0.177). 3 Body weights used in calculation of stocking rates and forage allowance were those estimated at the mid-points of the grazing seasons. a–c 1
Replanting done before the second grazing season negated evaluating year as a fixed effect; therefore, year and block were evaluated as random effects. Models analyzing nutritive values also evaluated sample collection number (5 sample dates in each year) within year and its interaction with the 2 treatments. Nutritive values were analyzed as repeated measures using the autoregression covariance structure (Littell et al., 2006). Nutritive-value data are presented in the results by date rate rather than sample collection number. Mean comparisons among treatments and their interactions were done using the PDIFF option of LSMEANS.
RESULTS AND DISCUSSION Treatment Targets Mean DMH averaged over the 2 grazing seasons for seed head–suppressed pastures with light and moderate grazing intensities were 7.2 ± 1.1 (forage mass = 2,263 ± 174 kg of DM/ha) and 6.1 ± 0.8 cm (forage mass = 2,276 ± 416 kg of DM/ha), respectively, and in nonsuppressed pastures were 8.7 ± 1.3 (forage mass = 3,081 ± 364 kg of DM/ha) and 7.3 ± 1.5 cm (2,539 ± 269 kg of DM/ ha), respectively. Overall, the targeted
DMH were met; however, mean DMH for the moderate grazing intensity of nonsuppressed pastures was slightly above the targeted range. Mean DMH for nonsuppressed pastures that were moderately grazed in the second grazing season was 7.6 ± 1.3 cm because of higher DMH during the early part of the season when there was considerable spot grazing that skewed the measures. Although herbage masses are numerically similar between the low and moderate grazing intensities for seed head–suppressed pastures, actual mean stocking rates and forage allowances with seed-head suppression differed (P < 0.05) between the 2 intensities (Table 1). Averaged over the 2 grazing seasons, there were fewer (P < 0.001) reproductive tillers in seed head–suppressed (21 ± 7 reproductive tillers/ m2) compared with nonsuppressed (90 ± 12 reproductive tillers/m2) pastures. Although Chaparral herbicide was effective in reducing seed heads, reproductive tiller densities were twice higher than the average of 10 reproductive tillers/m2 reported by Aiken et al. (2012) for endophyte-infected tall fescue treated with Chaparral. Ergovaline concentrations were zero for most of the whole tiller samples. Four of the 120 samples collected over both years had detectable ergova-
line concentrations, with the highest concentration in these 4 samples being 0.16 μg/g of DM. This provided strong evidence there was a small percentage of the fescue plants infected with the endophyte. Fribourg et al. (1991) concluded that signs of fescue toxicosis will not occur if infection percentages of tall fescue stands are less than 22%. Prolactin concentrations in steer serum did not differ (P = 0.398) between suppressed (241 ± 104 ng/ mL) and nonsuppressed (305 ± 102 ng/mL) pastures, which supports that only a small fraction of the plants were infected. Prolactin concentrations were highly variable and ranged in individual steers from 138 to 526 ng/mL. The greater densities of reproductive tiller in nonsuppressed pastures would likely have reduced prolactin concentrations in these pastures if a significant fraction of seed heads were infected and contained high concentrations of ergot alkaloids (Rottinghaus et al., 1991).
Pasture Productivity There was an interaction (P = 0.515) between seed-head suppression and grazing intensity on stocking rate (steers/ha; Table 1). Nonsuppressed pastures with the moderate
124 grazing intensity supported the most (P < 0.05) steers, whereas the lightly grazed, suppressed pastures supported the least (P < 0.05) number of steers. The lightly grazed, nonsuppressed and moderately grazed, suppressed pastures had similar (P = 0.515) stocking rates. However, there was no seedhead suppression × grazing intensity interaction (P = 0.177) with stocking rate on a BW basis. Greater BW was supported on nonsuppressed (1,519 ± kg of BW/ha) than suppressed (1,082 ± kg/ha) pastures and also was greater for moderately (1,497 ± kg of BW/ha) than lightly (1,105 ± kg of BW/ha) grazed pastures. Similar to Aiken et al. (2012), the tall fescue showed a lag in growth over the first few weeks after herbicide application. Although active growth was restored, the suppressed tall fescue appeared to remain behind in growth compared with nonsuppressed tall fescue. This resulted in nonsuppressed pastures supporting more steers, which was evident by the lightly grazed nonsuppressed pastures carrying a similar number of steers as moderately grazed suppressed pastures. Higher pasture stocking rates measured in experiments using put-andtake stocking are related to greater forage production (Mott, 1960). Wimer et al. (1986) reported that forage masses in smooth bromegrass pastures treated with mefluidide to
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suppress seed heads and grazed with cow-calf pairs (1.7 ha/cow-calf pair) averaged across the season approximately one-third of the forage produced by the control pastures. Aiken et al. (2012) reported less forage for seed head–suppressed pastures during the first year of the study but showed that warmer temperatures in the spring resulted in comparable levels of forage between herbicide treatments during the spring of the second year. Results of Aiken et al. (2012) and the present experiment strongly indicate seed head–suppressed tall fescue pastures will not support as many animals as nonsuppressed pastures. As mentioned previously, there were concerns of overgrazing in the seed head–suppressed pastures of endophyte-free fescue during the first growing season.
Botanical Composition Percentage tall fescue was less (P < 0.05) in suppressed (62.1 ± 2.3%) than nonsuppressed (69.7 ± 1.9%) pastures, and tended (P = 0.057) to contribute less DM in moderately (63.1 ± 2.2%) than lightly (68.9 ± 2.1%) grazed pastures (Figure 1). Orchardgrass DM percentage was not affected (P > 0.4) by seed-head suppression or grazing intensity. Percentage of the other species component was greater in suppressed (23.1 ±
Figure 1. Percentages and SE bars for endophyte-free tall fescue in the canopy DM in pasture mixture with orchardgrass and other grasses that were with or without chemical seed-head suppression and grazed with light or moderate grazing intensities.
3.1%) than nonsuppressed (15.9 ± 2.6%) pastures but was not affected (P = 0.382) by grazing intensity. This fraction was composed primarily of Kentucky bluegrass (Poa pratensis L.), crabgrass (Digitaria spp.), common chickweed [Stellaria media (L.) Vill.], purple deadnettle (Lamium purpureum L.), henbit (Lamium amplexicaule L.), and buckhorn plantain (Plantago coronopus L.) in 2011 and Kentucky bluegrass, crabgrass, and buckhorn plantain in 2012. Because the botanical composition of the pastures was determined on a DM basis, it was not surprising that Chaparral would lower the tall fescue biomass because of the inhibition of reproductive growth and its corresponding loss in DM. Chaparral herbicide does not affect reproductive development or growth of orchardgrass or Kentucky bluegrass (Aiken et al., 2012). Aiken et al. (2012) similarly reported a reduction in tall fescue of approximately 15% in Chaparral treatments within a tall fescue–Kentucky bluegrass mixture. Cattle also have been visually observed to selectively graze seed head–suppressed tall fescue (Aiken et al., 2012). This was indicated by the trend of reduced DM percentages of fescue in moderately grazed pastures.
Nutritive Values CP. There was an interaction (P < 0.05) between seed-head suppression and sample collection number in both years. Crude protein percentages in available forage was greater (P < 0.001) with suppression than without across all sampling dates in each year (Figure 2). Turner et al. (1990a) reported that N% in grazed endophyte-infected tall fescue that was treated with mefluidide to suppress seed heads was higher than in nonsuppressed fescue for samples collected from May to early July but not for those collected from late June to September. In the 2012 grazing season, CP in suppressed pastures was stable (P > 0.25) across sample dates, whereas it declined (P < 0.01) in nonsuppressed pastures between the
Seed-head suppression of tall fescue
Figure 2. Means and SE bars for sampling dates in 2011 (A) and 2012 (B) for CP percentages in available forage of pastures of endophyte-free tall fescue in mixture with orchardgrass and other grasses that were with or without chemical seed-head suppression. Asterisks above sampling dates denote differences (P < 0.05) between suppressed or unsuppressed pastures. Different letters between sampling dates for a given trend line are significantly different (P < 0.05).
23 April and 7 May sampling dates in the 2012 season. Crude protein in 2011 declined (P < 0.05) in suppressed and nonsuppressed pasture between the 29 May and 16 June sampling dates and stabilized for the remainder of the season.
Leaf blade and sheath components of suppressed vegetative tillers of fescue had greater percentages of CP than those components in nonsuppressed vegetative tillers across both years and sampling dates (Table 2). Leaf blades sampled 11 May in 2011
125 and leaf sheaths on 17 June in 2012 were similar in CP between suppressed vegetative tillers and nonsuppressed reproductive tillers, but CP was greater in suppressed blades and sheaths for the other dates. With the low density of reproductive tillers in suppressed pastures, protein and nonprotein N in vegetative tillers of suppressed pastures may have been conserved rather than being translocated to reproductive tissues. There was also an interaction (P < 0.05) between seed-head suppression and grazing intensity on CP of the available forage that was consistent (P = 0.104) across both years. Crude protein in suppressed pastures was not affected (P = 0.498) by grazing intensity, but plants in nonsuppressed pastures showed higher CP content (P < 0.001) in lightly (10.4%) than moderately (9.5%) grazed pastures. Low-quality stems (Collins and Fritz, 2003) likely had a greater presence in the nonsuppressed pastures under the light grazing intensity because of reduced grazing of plant growing points in the early season. Conversely, lack of difference between grazing intensities with seed-head suppression reflects a low density of stems through inhibition of stem elongation from the herbicide treatment. The difference in CP between seedhead suppression treatments was approximately 4 percentage units for the forage samples collected at each date. Increases in CP with suppression can be attributed to inhibition of tall fescue reproductive growth that will reduce the presence of stems and conserve nonprotein nitrogen in vegetative tissues that would normally be translocated to stem and seed tissues during reproductive development. Stems are inherently low in CP, whereas young vegetative tissues contain high levels of CP (Collins and Fritz, 2003). An increase in CP is common for most studies that have focused on the chemical suppression of reproductive growth (Wimer et al., 1986; Roberts and Moore, 1990; Moyer and Kelley, 1995). Aiken et al. (2012) reported higher forage CP concentrations in Chaparral-treated
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Table 2. Percentages of CP and IVDMD in the DM of leaf blades and sheath tissues of tall fescue in each year and sampling date that were separated from seed head–suppressed vegetative (Veg.) and nonsuppressed vegetative and reproductive (Rep.) tillers Seed-head suppression
Item
Leaf component
Year
Sample date
CP, % IVDMD, %
Blade Sheath Leaf Sheath
2011 2012 2011 2012 2011 2012 2011 2012
11 May 17 June 20 Apr 5 June 11 May 17 June 20 Apr 5 June 11 May 17 June 20 Apr 5 June 11 May 17 June 20 Apr 5 June
With
Without
Veg.
Veg.
Rep.
SEM1
18.9a 15.6a 16.2a 15.3a 11.2a 7.9a 11.1a 8.6a 83.7 75.4a 75.3 74.6a 81.0a 80.6a 72.3b 78.3a
15.9b 13.4b 11.1c 12.9b 8.7b 6.7b 6.8b 7.0b 83.7 72.3ab 71.2 71.2a 79.8a 79.7a 75.8a 77.3a
18.7a 11.1c 14.4b 8.0c 8.5b 8.0a 6.4b 6.1b 81.5 69.2b 75.1 57.2b 63.9b 56.4b 59.7c 52.4b
0.8 0.5 0.7 0.2 0.6 0.7 0.6 0.7 0.7 1.0 2.2 2.9 1.1 1.9 1.9 0.7
Means within leaf-component, year, and sampling-date groups with different superscripts differ statistically, P < 0.05. Standard error of the means were derived using the conservative calculation.
a–c 1
pastures, especially after the forage in the control pastures had entered the reproductive stages of development. IVDMD. Grazing intensity did not affect (P = 0.115) IVDMD of available forage, but there was a seed-head suppression × sample collection number interaction on IVDMD in both years. In vitro DM disappearance of available forage in 2011 was greater (P < 0.001) in suppressed than in nonsuppressed pastures across all sampling dates (Figure 3); however, a difference between suppressed and nonsuppressed pastures was detected (P < 0.05) in 2012 for only the last 3 sample dates. In support of these results, Turner et al. (1990a) reported greater in vitro OM digestibility of grazed endophyte-infected tall fescue treated with mefluidide than without the treatment from May to early August. Across sampling dates in 2011, IVDMD of available forage in suppressed pastures declined (P < 0.05) between the first and third sampling dates and was stable (P > 0.440) between the third and fifth dates,
whereas IVDMD for suppressed pastures in 2012 stabilized (P > 0.560) between the second and fifth sampling dates. For nonsuppressed pastures, IVDMD declined (P < 0.001) over the first 3 sample dates and then stabilized (P > 0.210) between the third and fifth sample dates. Leaf blades and sheaths of vegetative tillers in suppressed and nonsuppressed pastures did not differ (P > 0.12) in IVDMD for all sample dates in both years except for sheaths on 20 April in 2012 when IVDMD was greater for nonsuppressed than suppressed tillers. The components of vegetative tillers had greater (P < 0.05) IVDMD than reproductive tillers for all dates other than the first sampling dates for leaf blades when there were no differences between vegetative and reproductive tillers. The increases in IVDMD of available forage with seed-head suppression was therefore indicated due to a reduced presence of reproductive tillers that dampens the digestibility of the available forage. This dampening of
the IVDMD of reproductive tillers appeared to be from maturation of leaf blades during the season combined with maturation and possible movement of carbohydrates from sheath to stem and seed tissues during reproductive development. Aiken et al. (2012) reported that IVDMD of forage was similar between control and seed head–suppressed tall fescue pastures and that forage IVDMD decreased in the control pastures at the onset of reproductive growth. Lower IVDMD of forage in nonsuppressed pastures could be attributed to more reproductive growth. The concentrations of structural carbohydrates, such as cellulose and hemicelluloses, are greater in stems compared with the leaves and sheaths of vegetative tissues and are less degradable within the digestive system of the ruminants (Moore and Hatfield, 1994). Digestibility of reproductive tissues is further reduced by the continual deposition of these carbohydrates and lignin with the cell wall as the plant matures. Lignin, a phenolic conglom-
Seed-head suppression of tall fescue
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erate bound to hemicelluloses, reduces digestibility by impeding access to the carbohydrates by the degrading microbes (Jung and Deetz, 1993).
Steer Performance Steer ADG was greater (P < 0.05) with seed-head suppression than without (Table 3), and with no interaction between suppression and grazing intensity (P = 0.619). Higher steer BW gains on pastures of endophyte-infected tall fescue treated with mefluidide for seed-head suppression than those untreated was also reported by Turner et al. (1990b). The ADG response could be attributed to the higher CP and IVDMD of available forage in suppressed pastures (Table 2). The ultrasound-detected carcass-related traits reflected the higher weight-gain efficiency on seed head–suppressed pastures. Longissimus dorsi areas and subcutaneous fat over the rump and between the 12th and 13th ribs were greater (P < 0.05) in steers grazing suppressed pastures, which corresponded with visual observations of body size and condition. Aiken et al. (2002) concluded that ultrasonic measures of longissimus dorsi and subcutaneous fat are to be expected with positive growth rate responses to grazing treatments. The increased gain with seed-head suppression did not result in greater total BW gain per hectare, which tended to be higher (P < 0.068) without than with suppression. This was because stocking rates tended to be higher for nonsuppressed (5.0 ± 0.2 steers/ha) than suppressed (3.4 ± 0.2 steers/ha) pastures (Table 1).There was a strong tendency (P = 0.057) of an interaction between seed-head suppression and grazing intensity on average stocking rates (steers/ha) over the grazing seasons, but this was because stocking rates did not differ (P = 0.52) between the low grazing intensity with suppression (4.1 ± 0.3 steers/ha) and the moderate grazing intensity without seed-head suppression (4.1 ± 0.2 steers/ha). Stocking rates based on BW, however, were
Figure 3. Means and SE bars for sampling dates in 2011 (A) and 2012 (B) for IVDMD of available forage in pastures of endophyte-free tall fescue in mixture with orchardgrass and other grasses that were treated with or without chemical seed-head suppression. Asterisks above sampling dates denote differences (P < 0.05) between suppressed or unsuppressed pastures. Different letters between sampling dates for a given trend line are significantly different (P < 0.05).
greater for nonsuppressed (1,519 ± 75 kg of BW/ha) than suppressed (1,082 ± 110 kg of BW/ha), and there was a seed-head suppression × grazing intensity interaction (P = 0.177). Higher ADG for seed head–suppressed pastures apparently was not enough
to compensate for the lower numbers of steers that these pastures supported. Higher stocking rates with put-andtake experiments reflect greater forage production (Mott, 1960). Wimer et al. (1986) reported that forage
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Table 3. Mean weight gains and ultrasound-detected carcass-related traits for seed-head suppression of tall fescue and grazing intensity treatments for steers grazing mixed pastures of tall fescue, orchardgrass, and other forages Seed-head suppression Item1 ADG, kg/d BWG/ha, kg/ha LDA, cm2 Backfat, mm Rump fat, mm
With 0.94 (0.06) 253d (26) 56.3c (1.9) 3.6a (0.2) 3.2a (0.2) a
2
Grazing intensity
Without
Low
Moderate
0.79 (0.09) 304c (40) 50.9d (2.1) 3.2b (0.2) 2.9b (0.2)
0.93 (0.06) 251b (22) 54.0 (2.2) 3.4 (0.2) 3.1 (0.2)
0.80b (0.09) 305a (40) 53.1 (2.3) 3.3 (0.2) 3.0 (0.2)
b
a
Least squares means within treatment groups with different superscripts differ statistically, P < 0.05. Least squares means within treatment groups with different superscripts differ statistically, P < 0.10. 1 BWG/ha = BW gain per hectare; LDA = longissimus dorsi area. 2 Standard errors of the mean, shown in parentheses, were derived using the conservative calculation. a,b c,d
masses in smooth bromegrass pastures treated with mefluidide to suppress seed heads and grazed with cow-calf pairs (1.7 ha/cow-calf pair) averaged across the season approximately onethird of the forage produced by the control pastures. Aiken et al. (2012) reported less forage for seed head– suppressed pastures during the first year of the study but showed that warmer temperatures in the spring resulted in comparable levels of forage between herbicide treatments during the spring of the second year. Results of Aiken et al. (2012) and the present experiment strongly indicate seed head–suppressed tall-fescue pastures will not support as many animals as unsuppressed pastures. As mentioned previously, there were concerns of overgrazing in the seed head–suppressed pastures of endophyte-free fescue during the first growing season. Furthermore, Turner et al. (1990b) reported steers grazing endophyteinfected tall fescue treated with mefluidide to have 1.5-fold higher estimated OM intake than those grazing untreated tall fescue. Average daily gains were higher (P < 0.05) for steers on the low grazing intensity (3.5 ± steers/ha) treatment compared with the moderate grazing intensity (4.9 ± steers/ha). Average stocking rates on a BW basis for light and moderate grazing intensities were 515 ± 32 and 698 ± 49 kg of BW/
ha, respectively. This was a typical response for stocking rate studies (Mott, 1960; Jones and Sandland, 1974). Animals are able to selectively graze plant tissues that maximize the nutritive value (i.e., leaf tissue) of the forage under low grazing pressures, which results in a higher quality diet and better animal performance (Blaser et al., 1956; Mott; 1960; Bryant et al., 1965). Increases in stocking pressures create greater competition for available forage. Animals are then forced to be less selective in their diet and will consume plant tissues that are of lower nutritive value (i.e., stem), lowering animal performance (Blaser et al., 1956; Mott; 1960; Bryant et al., 1965).
IMPLICATIONS Chemical seed-head suppression improved forage nutritive value and increased steer ADG on pastures that contain 60 to 70% endophyte-free tall fescue in mixture with orchardgrass and other grasses. The ADG with seed-head suppression was 19% greater than without suppression. This was less than the 39% increase reported by Aiken et al. (2012) for steers grazing seed head–suppressed endophyteinfected tall fescue. This suggests that enhancement of forage quality through suppression of lower-quality reproductive tissues will benefit calf
performance, but the alleviation of toxic seed heads from endophyte-infected tall fescue could have a greater influence on steer performance.
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