Effects of stocking and supplementation rates on the performance of beef steers grazing mixed-grass prairie during the winter*

Applied Animal Science 35:641–651 https://doi.org/10.15232/aas.2019-01864 © 2019 American Registry of Professional Animal Scientists. All rights reserved.

SUSTAINABILITY AND INTEGRATED SYSTEMS: Original Research

Effects of stocking and supplementation rates on the performance of beef steers grazing mixed-grass prairie during the winter* Stacey A. Gunter,† PAS USDA, Agricultural Research Service, Southern Plains Range Research Station, Woodward, OK 73801-5415

ABSTRACT Objective: The objective was to evaluate the effects of stocking and supplementation rates on steers grazing native mixed-grass prairie during the winter. Materials and Methods: Sixteen pastures (10 to 21 ha each) were selected. Treatments were arranged in a 3 × 2 factorial: the first factor was an annual stocking rate of 39.4, 33.2, and 29.7 animal-unit-d/ha for 88 d; the second factor was supplementation at 0.9 or 1.4 kg/steer per day. Steers were fed a 43% CP cottonseed meal–based pellet. Data were analyzed with an ANOVA, and the pasture was the experimental unit. Results and Discussion: The effect of stocking rate on ADG and BW per steer over the 88-d grazing period interacted (P < 0.01) with supplementation rate; ADG and total BW gain per steer (kg) responded quadratically to stocking rate with 0.9 kg/d of supplementation (P < 0.01; 0.42, 0.57, and 0.47; 40, 48, and 40, respectively), but with 1.4 kg/d of supplement, they were unaffected (P ≥ 0.34; 0.56, 0.53, and 0.51; 48, 48, and 46, respectively). Body weight gain per hectare (kg) tended (P = 0.08) to interact between stocking and supplementation rates, and at 0.9 kg/d of supplement, the BW gain per hectare increased quadratically (P < 0.01; 24, 32, and 32, respectively) in response to increasing stocking rate. However, with 1.4 kg/d of supplement, BW gain per hectare responded linearly (P < 0.01; 29, 31, and 35, respectively). Implications and Applications: Optimal supplementation rates with high-protein feeds interacts with stocking rate. At lower stocking rates, less supplement seems The author declares no conflict of interest. *Mention of trade names or commercial products in this article is solely for providing specific information and does not imply recommendation or endorsement by the USDA. The USDA prohibits discrimination in all its programs and activities based on race, color, national origin, age, disability, and where applicable, sex, marital status, familial status, parental status, religion, sexual orientation, genetic information, political beliefs, reprisal, or because all or part of an individual’s income is derived from any public assistance program. †Corresponding author: stacey.gunter@​usda​.gov

to be most beneficial. However, at higher stocking rates, more supplement is justified as evidenced by the linear increase of BW gain per hectare. Key words: cattle, grazing, growth, rangeland, sand sagebrush

INTRODUCTION Mixed grasslands with an overstory of sand sagebrush (Artemisia filifolia Torr.) extends across approximately 6 million hectares of sandy soils on the Southern Plains (Berg, 1994). This sand sagebrush grassland occurs along the northern side of major rivers that flow diagonally across the Southern Plains. Research on this type of rangelands has primarily focused on sagebrush control methods (McIlvain and Savage, 1949; Bovey et al., 1981), grazing management (Sims et al., 1976; Sims and Gillen, 1999; Gillen and Sims, 2004), and wildlife habitat (Cannon and Knopf, 1981; Rodgers and Sexson, 1990; Patten et al., 2005). A large portion of sand-sagebrush grassland is managed primarily for maximum livestock production per hectare. Stocking rate is the primary determinant of animal BW gain and enterprise profitability because of the stocking rate effects on herbage allowance and dietary quality (Sims and Gillen, 1999; Gunter et al., 2005; Torell et al., 2010). However, supplementation with protein or energy may augment animal performance at any stocking rate and permit for greater economic return to producers (Shoop and McIlvain, 1971a,b; McCollum and Horn, 1990; Moore et al., 1999). Development of management systems to lower wintering costs and increase the net return to producers ranching on the Southern Plains could allow them to graze their rangelands less heavily during the physiologically critical summer months (McIlvain and Shoop, 1962). Increasing stocking rates during the winter, while warm-season forages are dormant, and increasing supplementation rates to maintain animal performance would allow for economies of scale for fixed cost and decreasing the cost of BW gain (Workman, 1986). It is believed that optimal supplementation rate is dependent on the stocking rate, but little research has been completed regarding

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this question with growing cattle (Beck et al., 2008), especially on dormant rangelands. Because of the scarcity of information addressing the probable interactions between protein supplementation and stocking rate with growing cattle grazing dormant rangeland, the objectives of this experiment were to determine the effects of stocking and supplementation rates on the performance of beef steers grazing dormant mixedgrass prairie and their interaction.

MATERIALS AND METHODS This study was conducted from 2009 through 2011 at the Southern Plains Experimental Range (36°35′N, 99°35′W; elevation 630 m) of the USDA, Agricultural Research Service, near Fort Supply, Oklahoma. All animal procedures were conducted in accordance with the recommendations of the Consortium (FASS, 1999), and procedures were approved by the Southern Plains Range Research Station Animal Care and Use Committee.

Site and Experimental Design The regional climate is continental with an average annual precipitation of 627 mm and with 72% of the precipitation falling during the April-to-September growing season (Gillen and Sims, 2006; Gunter et al., 2012). Average monthly mean temperatures are 2.3°C in January and 28°C in July. Minimum and maximum recorded temperatures are −28°C and 45°C, respectively. This region consists of gently rolling and stabilized sand dunes frequently interspersed with areas of heavier textured soils that have no well-defined drainage patterns (Berg, 1994). The vegetation is dominated by a mixture of tall, mid, and short warm-season grasses and forbs, and sand sagebrush. The native vegetation of the Southern Plains mixed-grass prairie is within the sandsage–bluestem (Artemisia–Andropogon) prairie type described by Küchler (1964). Pratt soils (sandy, mixed, mesic Psammentic Haplustalfs) are on lower slopes and more level areas; Tivoli soils (mixed, thermic Typic Ustipsamments) occur on upper slopes (Berg, 1994). Treatments were applied in this study in a 3 × 2 factorial arrangement. In applying the first factor, the 16 pastures, that ranged in size from 10 to 21 ha, were initially surveyed annually in 2003 through 2006 for sand sagebrush canopy cover by the line-transect method of Parker and Savage (1944) and were found to range from 1 to 19% (Thacker et al., 2012). Stocking rates for the previous 5 yr (2003 to 2006) were and are considered moderate [ranged from 47 to 69 animal-unit-d (AUD)] for the region (Shoop and McIlvain, 1971b; Sims and Gillen, 1999), and the cattle grazed from January through August. The pastures were then stratified by percentage of sand sagebrush cover and stocking rate treatments and were assigned to high (n = 4 pastures/yr), medium (n = 6 pastures/yr), and low (n = 6 pastures/yr) stocking rates. Because metabolic BW is

linearly related (r = 0.98) to heat of production (Kleiber, 1947) and forage demand (NASEM, 2016; Almeida et al., 2019), initial metabolic BW of steers was divided by the metabolic BW of the standard 454-kg cow (Vallentine, 1965) times the daily stocking density per hectare, scaling the stocking rate estimates to the standard animal unit as defined by Allen et al. (1992). Steer numbers per pasture varied among years because pastures were randomly assigned annually, but the initial stocking rates on a perhectare basis averaged 29.7, 33.2, and 39.4 AUD annually for the low, medium, and high stocking rates, respectively. In applying the second factor, supplementation rate was randomly applied within stocking rate. Supplementation rates and CP concentration was selected based on previous research from this location (Savage and Heller, 1947; McIlvain and Shoop, 1962; Shoop and McIlvain, 1971a; Gadberry et al., 2012). Hence, during the entire grazing period steers were fed (DM basis) a 43% CP cottonseed meal–based (15.7% ADF) supplement (1.9-cm-diameter pellet) at rates of either 0.9 kg (n = 8 pastures per year) or 1.4 kg (n = 8 pastures per year) per steer per day. The steers had continuous access to 20-kg blocks of NaCl but had access to no other mineral supplement as they were deemed unnecessary based on the research of Savage and Heller (1947). Water was always available in 7.6-m-diameter troughs (0.61-m deep) filled by windmills over shallow wells (sulfates ≤370 mg/kg).

Cattle Measurements All pastures were grazed by crossbred beef steers (220 ± 6.7 kg; British × Continental) at stocking densities as described in the pasture treatments. Each year, steer and bull calves were amalgamated from sale barns near Kaufman, Texas, and received at a commercial feedlot for 3 to 6 wk. During receiving, bulls were castrated and horns tipped; all calves were treated for internal and external parasites (Cydectin; Fort Dodge Animal Health, Overland Park, KS); implanted (Ralgro; Schering-Plough Animal Health, Union, NJ); injected with a modified live 4-way vaccine for infectious bovine rhinotracheitis, bovine respiratory parainfluenza-3, bovine respiratory syncytial virus, and bovine virus diarrhea; and injected with a 7-way clostridial vaccine. Calves were revaccinated before leaving the feedlot. Calves were then shipped to the Southern Plains Experimental Range in mid-November, where they were individually weighed, number branded, and then placed on dormant pasture and fed a 43% CP cottonseed meal–based supplement at a rate of 0.68 to 0.91 kg/steer per day as suggested by McIlvain and Shoop (1962) and Shoop and McIlvain (1971a) until the experiment started (6 to 8 wk). Starting in late-January, steers were gathered in the afternoon, placed in a pen without feed or water until the next morning (approximately 17 h) to control for gastrointestinal fill (Aiken and Tabler, 2004), and then weighed individually. After weighing, steers were stratified by BW then randomly assigned and sorted to 1 of the

Gunter: Supplementation on mixed-grass prairie

16 pastures. In mid-March and at the end of the grazing period in early-April, steers were reweighed after an overnight shrink as previously described. Average weigh dates over the 3-yr period for the beginning (late-January), intermediate (mid-March), and ending (late-April) of the grazing period were January 27, March 12, and April 26, respectively.

Herbage Measurements Herbage mass in each pasture was estimated in lateJanuary, mid-March, and late-April near each weigh date by clipping forage to ground level inside 0.1-m2 frames (n = 40) at fixed transects arranged in a grid formation to encompass all vegetation and soil types each time of clipping using a global position system (±10 m; Garmin GPS 72, Olathe, KS). When a transect position was located and if a sand sagebrush plant was in the frame, all herbage was harvested, and the sand sagebrush plant was left unclipped. Herbage samples were weighed individually in the field with a 30-g spring scale (Model No. 20030, Pesola, Feusisberg, Switzerland) and then dried (60°C) for 48 h to determine herbage DM. After herbage samples had been weighed, the samples were hand separated into leaves and stems. The leaves from both the graminoids and forbs were composited within pasture, ground to pass a 1-mm screen (Thomas A. Wiley Laboratory Mill, Model 4, Thomas Scientific, Swedesboro, NJ), and analyzed for total N with a rapid combustion analyzer (Vario MAX CN; Elementar Amerias Inc., Mt. Laurel, NJ) and in vitro OM disappearance (IVOMD) according to Tilley and Terry (1963) as modified by White et al. (1981). Nitrogen concentration was multiplied by 6.25 to estimate CP. The IVOMD:CP ratio was calculated by dividing the percentage of IVOMD by the percentage of CP on a DM basis as described by Gunter et al. (1995). Composite samples were analyzed for DM and ash (AOAC International, 2000) and for NDF and ADF (Van Soest et al., 1991) including amylase digestion (NDF only) by the batch procedures outlined by Ankom Technology Corp. (Fairport, NY).

Statistical Analysis Pasture means of forage mass and nutritive values, initial BW, intermediate BW, final BW, ADG, ADG for each period and over the entire grazing season, BW gain per hectare, and BW gain per steer were analyzed by ANOVA using the GLIMMIX Procedure in SAS (SAS Institute Inc., Cary, NC). Early- and later-winter ADG were calculated by subtracting mid-March or late-April from late-January or mid-March, respectively, and dividing by the number of days in the grazing period. The fixed effects in the model were stocking rate, supplementation rate, and their interactions; the random effects were year and pasture (Bello et al., 2016). Because an objective of this experiment was to access the effectiveness of these treatments over multiple production cycles, year was clas-

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sified as a random effect so inferences could be projected over multiple years of adoption (Gbur et al., 2012). If a significant F-test was detected (P < 0.10) for the interaction between stocking and supplementations rates, data were re-analyzed for stocking rate effects within supplementation rate. Least squares means for stocking rates were analyzed for linear and quadratic effects by supplementation rate using a set of contrasts. Because of variations in initial BW of the cattle and because they were randomly assigned to pastures, stocking rates were not as evenly spaced as originally designed. Hence, the contrast coefficients were constructed using procedures described by Robson (1959) for unequally spaced treatments. Differences among least squares means were deemed significant at P ≤ 0.05, and tendencies are mentioned when P ≤ 0.10. If a linear or quadratic effect was deemed significant (P < 0.05) or tended to be significant (P ≤ 0.10) for the stocking rate effect within supplementation rate, response variables were regressed on stocking rate, and stocking rate-squared if it was a quadratic response, as described by Neter et al. (1989) using the REG Procedure (SAS Institute Inc.).

RESULTS AND DISCUSSION Precipitation Annual precipitation at the Southern Plains Experimental Range weather station (1.7 km west of the pastures) over the 3-yr period of the experiment (June through May; 427 mm) averaged 31% less than the long-term average (627 mm). In the later 2 yr (2010 and 2011) of the experiment, precipitation was more than 46% less than the longterm average, whereas in the first year (2009), it was near (102%) the long-term average (Figure 1). This extremely large variation noted in annual precipitation among years is not unusual for this region of the United States (Gillen and Sims, 2006; Gunter et al., 2012; Ponce Campos et al., 2013) and has been found to be the normal characteristic of the annual precipitation for the last 1,012 yr accessed via tree-ring chronology (Stambaugh et al., 2011). However, all 3 yr had a normal distribution of precipitation within each year with dry winters and the majority of precipitation occurring in the spring and early summers compared with the previous 20 yr that occurred before 2009 when this experiment was initiated (Moffet and Reuter, 2017).

Herbage Characteristics Standing herbage DM in the pastures at the beginning, intermediate, and end of the grazing periods did not (P ≥ 0.27) interact among stocking and supplementation rates (Table 1); further, standing herbage DM was not affected by supplementation rate (P ≥ 0.46) or stocking rate (P ≥ 0.27). Sims and Gillen (1999) reported in a long-term stocking rate experiment that over 10 yr, the titrated effects of increasing stocking rate from 41 to 82 AUD/ha in

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Figure 1. Accumulated precipitation by year from June 2008 through May 2011 at the Southern Plains Experimental Range near Ft. Supply, Oklahoma.

a 320-d grazing period (November 13 to September 29) still had little effect on total standing herbage mass on the site even though the differences in stocking rate did result in differences in steer ADG (Sims and Gillen, 1999). In a pair of experiments at the Southern Plains Experimental Range, Gillen and Sims (2004) reported that stocking of cow-calf pairs from 45 to 87 AUD/ha in a year-long grazing period still had little effect on total standing herbage mass on the site even though the stocking rate increase did linearly decrease calf weaning weight (Gillen and Sims, 2002). In an experiment reported by Gillen et al. (2000) where growing cattle were grazing mixed-grass prairie in Bessie, Oklahoma (35°50′N, 99°8′W) during the growing season, standing herbage mass decreased by 14.4 kg/ha for each AUD increase in stocking rate. When growing cattle were stocked on growing tallgrass prairie using a continuous grazing system with stocking rates ranging from 51.5 to 89.8 AUD/ha, standing herbage mass decreased by increasing stocking rate (Gillen et al., 1998). The lack of differences reported relative to stocking rate responses might be associated with the extreme heterogeneity of the canopy structure associated with sand-sagebrush communities (Gillen and Sims, 2006; Thacker et al., 2012) and level of replication in sampling. Crude protein concentration and the IVOMD in the leaves gleaned from the herbage samples at the beginning, intermediate, and end of the grazing periods did not (P ≥ 0.35) interact among stocking and supplementation rates (Table 2); further, the CP concentration and IVOMD was

not affected by supplementation rate (P ≥ 0.39) or stocking rate (P ≥ 0.52). The CP and IVOMD concentrations in the herbage samples are similar to values reported by Gadberry et al. (2012) for cows and their nursing calves grazing similar range site at the Southern Plains Experimental Range. Using dietary concentration as a guide, the concentration of CP in the leaves was below the cattle’s requirement for CP of 9.4% of DM for a 225-kg large-framed steers with a targeted BW gain of 0.45 kg/d (NRC, 1984). Also, the IVOMD requirement is suggested to be 56% of DM assuming a DMI of 5.58 kg/d (NRC, 1984). The cattle gained BW more quickly than a digestion model predicted (NASEM, 2016), but these discrepancies among predicted performance and actual BW gains are not uncommon, especially with lower quality herbages (Villalobos et al., 1997; Lardy et al., 2004). Angell et al. (1986) reported that IVOMD concentration in the diets of grazing cattle only accounts for 66% in the variation noted in the growth rates of cattle grazing Gulf cordgrass (Spartina spartinae) pastures. The ratio of IVOMD:CP in leaves gleaned from the herbage samples at the beginning, intermediate, and end of the grazing periods, similar to CP and IVOMD, did not (P ≥ 0.47) interact among stocking and supplementation rates (Table 2); further, the NDF and ADF concentrations were not affected by supplementation rate (P ≥ 0.35) or stocking rate (P ≥ 0.50). The ratio of IVOMD:CP in leaves averaged 5.8 and ranged from 5.2 to 6.1. A ratio of IVOMD:CP between 4.0 and 4.5 is considered balanced

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in ruminal available N to ruminal available energy in the diets of ruminants (Gunter et al., 1995). The ratio of IVOMD:CP for all leaf samples was 5.2 or greater. Based on the suggestion of Moore et al. (1999) and McCollum and Horn (1990), a supplement high in ruminally degradable N should increase DMI, extent and rate of digestion, and ADG (McCollum and Galyean, 1985; Villalobos et al., 1997). Neutral and acid detergent fiber concentrations in leaves gleaned from the herbage samples at the beginning, intermediate, and end of the grazing periods did not (P ≥ 0.22) interact among stocking and supplementation rates (Table 3); further, the NDF and ADF concentrations were not affected by supplementation rate (P ≥ 0.51) or stocking rate (P ≥ 0.51). The NDF and ADF concentrations in the herbage samples were similar to values reported by Gadberry et al. (2012) for cattle grazing a similar range site at the Southern Plains Experimental Range. During winter on native rangelands, grazing normally does not affect the detergent fiber concentrations in standing herbage mass as a result of selective grazing because stocking rates are normally lower and herbage allowance is more liberal than normally seen with pasture systems (Cochran et al., 1986; Barton et al., 1992; Johnson et al., 1998).

Cattle Performance Initial steer BW did not differ (P = 0.89) among stocking rates, between supplementation rates (P = 0.95), or interact among the treatments (P = 0.96); however, intermediate (P = 0.02) and final (P = 0.09) BW of the steers did

interact or tend to interact, respectively, between stocking and supplementation rates (Figure 2). When steers were supplemented at 0.91 kg/d, their intermediate BW decreased linearly (P < 0.01) in response to increasing stocking rates; however, when steers were supplemented at 1.4 kg/d, their intermediate BW was unaffected (P ≥ 0.46) by the increasing stocking rate. When steers were removed from the pastures in late-April, their final BW tended (P = 0.09) to interact between stocking and supplementation rates. When steers were supplemented at 0.9 kg/d, their final BW responded quadratically (P < 0.01) to increasing stocking rates; further, when steers were supplemented at 1.4 kg/d, their final BW was unaffected (P ≥ 0.41) by the increasing stocking rate. At the 0.9 kg/d supplementation rate, each AUD per hectare increase in stocking rate decreased (P = 0.02) final BW 5.961 kg at an increasing rate (0.078 × BW2) over the 88 d of grazing [root mean squared error (RMSE) = 9.431], where at 1.4 kg/d of supplement stocking did not relate to final BW (P ≥ 0.21). Hence, the higher rate of supplementation was able to compensate for the inferred decreased herbage allowance and maintain growth rate despite the increase in stocking rate. The early-winter ADG of the steers, late-January to mid-March (44 d), did not differ (P = 0.13) among stocking rates or interact between stocking and supplementation rates (P = 0.37). However, the main effect of supplementing at 1.4 kg/d increased (P = 0.01) ADG (0.53 kg) by 9.5% compared with supplementation at 0.9 kg/d (0.47 kg, SE = 0.019; Table 4). However, late-winter ADG, from mid-March to late-April (44 d), and during the entire graz-

Table 1. Available herbage mass in native mixed-grass prairie pastures grazed by stocker cattle during the winter (late-January through late-April, 88 d) on the Southern Plains Experimental Range north of Ft. Supply, Oklahoma, in 2009, 2010, and 2011 Stocking rate2 Item and supplementation rate1 Beginning herbage mass (d 0), kg of DM/ha   0.9 kg/d   1.4 kg/d  SE Intermediate herbage mass (d 44), kg of DM/ha   0.9 kg/d   1.4 kg/d  SE Ending herbage mass (d 88), kg of DM/ha   0.9 kg/d   1.4 kg/d  SE

Low  

977 867 246   1,183 1,045 324   886 800 246

Medium  

791 674 255   1,011 1,039 329   663 1,012 258

High  

729 822 248   982 1,035 324   896 886 248

SE  

255 250     329 326     258 251  

A 43% CP (DM basis) cottonseed meal–based supplement (1.9-cm-diameter pellet) was fed 3 d weekly at rates that averaged either 0.9 or 1.4 kg of DM/steer daily. 2 Low = an initial stocking rate of 29.7 animal-unit-d/ha annually, medium = an initial stocking rate of 33.2 animal-unit-d/ha annually, and high = an initial stocking rate of 39.4 animal-unit-d/ ha annually. 1

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Table 2. Crude protein and in vitro OM digestibility (IVOMD) concentrations in leaves gleaned from the herbage samples from native mixed-grass prairie pastures grazed by stocker cattle during the winter (lateJanuary through late-April, 88 d) on the Southern Plains Experimental Range north of Ft. Supply, Oklahoma, in 2009, 2010, and 2011 Item, period, and supplementation rate1 CP, % of OM  Beginning   0.9 kg/d   1.4 kg/d   SE  Intermediate   0.9 kg/d   1.4 kg/d   SE  Ending3   0.9 kg/d   1.4 kg/d   SE IVOMD, %  Beginning   0.9 kg/d   1.4 kg/d   SE  Intermediate   0.9 kg/d   1.4 kg/d   SE  Ending   0.9 kg/d   1.4 kg/d   SE IVOMD:CP ratio  Beginning   0.9 kg/d   1.4 kg/d   SE  Intermediate   0.9 kg/d   1.4 kg/d   SE  Ending   0.9 kg/d   1.4 kg/d   SE

Stocking rate2 Low    

6.0 6.1 0.40   6.0 6.2 0.59   8.4 8.1 0.81     38.6 37.1 1.45   35.7 36.1 2.50   43.8 42.2 2.27     6.5 6.1 0.31   6.1 5.9 0.53   5.3 5.3 0.38

Medium    

6.4 6.4 0.43   6.8 6.0 0.63   8.5 7.9 0.85     37.6 36.5 1.64   37.6 34.8 2.63   43.8 41.5 2.48     5.9 5.8 0.33   5.7 5.9 0.57   5.2 5.5 0.40

High    

6.2 5.7 0.40   6.0 6.1 0.60   7.3 7.3 0.81     36.9 36.1 1.45   35.2 37.4 2.52   39.1 41.4 2.29     6.0 6.4 0.31   6.0 6.3 0.54   5.4 5.7 0.37

SE    

0.43 0.41     0.63 0.61     0.85 0.82       1.64 1.53     2.63 2.55     2.48 2.35       0.33 0.32     0.57 0.55     0.40 0.39  

A 43% CP (DM basis) cottonseed meal–based supplement (1.9-cm-diameter pellet) was fed 3 d weekly at rates that averaged either 0.9 or 1.4 kg of DM/steer daily. 2 Low = an initial stocking rate of 29.7 animal-unit-d/ha annually, medium = an initial stocking rate of 33.2 animalunit-d/ha annually, and high = an initial stocking rate of 39.4 animal-unit-d/ha annually. 3 Linear effect (P ≤ 0.08) across supplementation rates. 1

ing period (88 d), did interact (P < 0.01) between stocking and supplementation rates (Table 4). So, cattle supplemented at 0.9 kg/d had an ADG during the late-winter and over the entire grazing period that increased quadratically (P < 0.01) with increasing stocking rate. Steers grazing the moderately stocked pastures probably removed a greater portion of the of the obstructing dormant herbage mass, hence allowing cattle greater access to the preferred cool-season, annual grasses normally present in the spring as suggested by the increases noted in CP concentration during the last herbage sampling event conducted in April (Drescher, 2003). Steers supplemented at 1.4 kg/d had an ADG in the late-winter grazing period that tended (P = 0.09) to decrease with increasing stocking rates. But, the ADG across the entire grazing period interacted (P < 0.01) between stocking and supplementation rates. Steers supplemented at 0.9 kg/d showed a quadratic response (P < 0.01) in ADG as stocking rate was increased. When steers were supplemented at 1.4 kg/d, ADG did not differ (P ≥ 0.19) because of increased stocking rates similar to the responses noted for intermediate and ending BW (Table 4). Because of the quadratic response, each unit increase in AUD decreased (P = 0.01) late-winter and overall ADG by −0.155 and −0.0921 kg and increased (P = 0.01) the rate by 0.00199 and 0.00115 kg2 over the 44-d and 88-d grazing periods (RMSE = 0.2414 and 0.1162), respectively. There was a linear response with 1.4 kg/d of supplementation in late-winter and overall as it decreased (P < 0.01) ADG −0.155 and −0.0149 kg during the 44-d and 88-d grazing periods (RMSE = 0.2443 and 0.1403), respectively. Total BW gain per steer and steer BW gain per hectare tended to interact (P ≤ 0.09) between stocking and supplementation rates (Table 4). Hence, total BW gain per steer when supplemented at 0.9 kg/d responded quadratically (P < 0.01) to an increased stocking rate over the 88-d grazing period. Unlike the lower supplementation rate, steers supplemented at 1.4 kg/d did not differ (P ≥ 0.41) in total BW gain per steer because of increasing the stocking rate. Steer BW gain per hectare when supplemented at 0.91 kg/d responded quadratically (P < 0.01) to increasing stocking rates over the 88-d grazing period. However, steer BW gain per hectare when steers were supplemented with 1.4 kg/d increased linearly (P < 0.01) in response to the increased stocking rate. Within the 88-d grazing period, the regression of the dependent responses on stocking rate at 0.9 kg/d of supplementation resulted in a quadratic response, and each unit increase in AUD decreased (P ≤ 0.08) total BW gain and BW gain per hectare by −5.001 and −2.632 kg and increased (P = 0.01) the rate by 0.0380 and 0.0408 BW2 (RMSE = 9.649 and 7.814), respectively. With the linear response associated with 1.4 kg/d of supplementation, total BW gain and BW gain per hectare decreased (P < 0.09) −0.431 and −0.109 kg during the grazing periods (RMSE = 11.145 and 7.773), respectively.

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Table 3. Detergent fiber concentrations in leaves gleaned from the herbage samples from native mixed-grass prairie pastures grazed by stocker cattle during the winter (late-January through late-April, 88 d) on the Southern Plains Experimental Range north of Ft. Supply, OK in 2009, 2010, and 2011 Stocking rate2

Item, period, and supplementation rate1 NDF, % of OM  Beginning   0.9 kg/d   1.4 kg/d   SE  Intermediate   0.9 kg/d   1.4 kg/d   SE  Ending3   0.9 kg/d   1.4 kg/d   SE ADF, % of OM  Beginning   0.9 kg/d   1.4 kg/d   SE  Intermediate   0.9 kg/d   1.4 kg/d   SE  Ending   0.9 kg/d   1.4 kg/d   SE

Low    

74.1 73.9 1.55   74.8 74.4 2.06   70.8 71.7 1.31     49.7 49.2 1.02   50.6 49.9 1.22   46.9 46.7 1.09

Medium    

73.5 72.7 1.61   73.0 74.7 2.09   70.2 72.2 1.48     49.3 48.2 1.12   49.2 50.0 1.27   46.5 47.6 1.20

High    

73.1 74.5 1.55   73.7 74.2 2.06   73.8 73.3 1.38     48.0 50.1 1.03   49.8 50.0 1.22   48.0 48.3 1.10

SE    

1.61 1.58     2.09 2.07     1.48 1.42       1.12 1.07     1.27 1.24     1.20 1.13  

A 43% CP (DM basis) cottonseed meal–based supplement (1.9-cm-diameter pellet) was fed 3 d weekly at rates that averaged either 0.9 or 1.4 kg of DM/steer daily. 2 Low = an initial stocking rate of 29.7 animal-unit-d/ha annually, medium = an initial stocking rate of 33.2 animal-unit-d/ha annually, and high = an initial stocking rate of 39.4 animal-unit-d/ ha annually. 3 Linear effect (P ≤ 0.08) across supplementation rates. 1

The response of steers to increasing stocking rates on intermediate and final BW, ADG, and BW gain per hectare reflected similar responses reported by Lauchbaugh (1957) for steers grazing shortgrass prairies during the winter at a location in Kansas 276 km directly north of the Southern Plains Experimental Range. Research reported from our location has shown that increasing stocking rate diminishes individual animal performance but increases BW gain per hectare for growing cattle in season-long grazing (Shoop and McIlvain, 1971a; Sims and Gillen, 1999), and this classic linear response to increasing stocking rate (MacLeod and McIntyre, 1997) has been noted by other scientists with alternate forage systems (Klipple and Costello, 1960; Riewe, 1961; Petersen et al., 1965; McCollum et al., 1999). Even though increasing stocking rate increases BW gain per hectare and probably net return

to the enterprise in years with normal precipitation, this practice increases risk to the enterprise, especially during times of drought (Klipple and Costello, 1960; Shoop and McIlvain, 1971b; MacLeod and McIntyre, 1997; Irisarri et al., 2019). Research has shown that using higher stocking rates during times of drought is even more detrimental to animal performance than conservative stocking rates and produces have greater financial losses to the cattle enterprise (Klipple and Costello, 1960; Shoop and McIlvain, 1971b; Torell et al., 1991). The desire to maintain future herbage production is significant to producers using native rangeland and a concern because heavier stocking rates decrease future herbage production (Adiku et al., 2010) as a result of damage to plant roots (Pulido et al., 2017) and a decrease in water infiltration (Rhoades et al., 1964; Ahmed et al., 1987) resulting from soil compaction. Fur-

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ther, mechanical intervention to renovate rangelands from overgrazing and soil compaction are often not an option and is usually uneconomical compared with long-term appropriate stocking rates (Vallentine, 1989). The overall ADG by the steers differed by stocking rate, with quadratic response and peak ADG occurring at the medium stocking rate when supplemented at 0.9 kg/d, where with 1.4 kg/d supplementation ADG did not differ because of increased stocking rates. Research reports from other locations have shown similar responses in ADG to changes in stocking rates (Lauchbaugh, 1957; Cochran et al., 1986; Beaty et al., 1994; Villalobos et al., 1997). Protein supplementation provides ruminal degradable protein to a basal diet that is deficient for effective ruminal microbial digestion of fiber and microbial protein production (Kartchner, 1980; Nocek and Russell, 1988; Sauvant and Noziere, 2016). Body weight gain per hectare is normally a more sensitive response variable in stocking and supplementation rate experiments, probably because of the multiplicative effect of several cattle and days per experimental unit. The responses to stocking and supplementation rate changes noted in our experiment are quite typical compared with other experiments conducted on the Great Plains (Klipple and Costello, 1960; McIlvain and Shoop, 1962; Shoop and McIlvain, 1971a; Grings et al., 1994) and other locations where warm-season grass dominates the landscape (Poppi and McLennan, 1995; Njoya, 1997; Gunter et al., 2005). However, an important consideration

is where the optimal supplementation rate for a particular forage allowance is suggested in this experiment because of the stocking and supplementation rate interactions noted for various performance responses (Table 4). The response to supplementation was dependent on stocking rate, and cattle at higher stocking rates, in general, benefitted from greater amounts of supplement as demonstrated by the declines in animal performance parameters where supplement at 0.9 kg/d was offered.

APPLICATIONS Optimal supplementation rates and performance response with high-protein feeds are related to stocking rate. At lower stocking rates, less supplement seemed to be most beneficial, whereas higher rates of supplementation seemed unfruitful. However, at higher stocking rates where forage allowance is more restrictive, more protein supplement is justified, evidenced by the linear increase noted in BW gain per hectare. However, conservative stocking rates are advised when planning a grazing program because of the possibility of drought and the overstocking effects on plant and soil health.

ACKNOWLEDGMENTS This research was funded by the USDA, Agricultural Research Service (Washington, DC) within the project Sustaining and enhancing southern plains rangeland and

Figure 2. Body weight of stocker cattle grazing mixed-grass prairie during the winter on the Southern Plains Experimental Range north of Ft. Supply, Oklahoma, starting Beginning (January 27), Intermediate (March 12), through Ending (April 26) in 2009, 2010, and 2011. aA 43% CP (DM basis) cottonseed meal–based supplement (1.9-cm-diameter pellet) was fed 3 d weekly at rates that averaged either 0.9 or 1.4 kg of DM/steer daily. bLow SR = an initial stocking rate of 29.7 animal-unit-d/ha annually, Medium SR = an initial stocking rate of 33.2 animal-unit-d/ha annually, and High SR = an initial stocking rate of 39.4 animal-unit-d/ha annually. cSupplementation rate interacted (P ≤ 0.01) with stocking rate, so each level of supplementation was analyzed separately. d Supplementation rate tended (P ≤ 0.09) to interact with stocking rate, so each level of supplementation was analyzed separately. e Body weight responded linearly to SR (P ≤ 0.01) at 0.9 kg/d of supplement. fBody weight responded quadratically to SR (P ≤ 0.01) at 0.9 kg/d of supplement.

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Table 4. Performance of stocker cattle grazing mixed-grass prairie during the winter (lateJanuary through late-April, 88 d) on the Southern Plains Experimental Range north of Ft. Supply, Oklahoma, in 2009, 2010, and 2011 Stocking rate2 Item and supplementation rate1 ADG in early winter, kg   0.9 kg/d   1.4 kg/d  SE ADG in late winter,3 kg   0.9 kg/d4   1.4 kg/d5 ADG over entire winter,3 kg   0.9 kg/d4   1.4 kg/d BW gain/steer,6 kg   0.9 kg/d4   1.4 kg/d BW gain/ha,6 kg   0.9 kg/d4   1.4 kg/d5

Low  

0.50 0.53 0.027   0.35 0.56   0.42 0.56   40 48   24 29

Medium  

0.49 0.52 0.031   0.62 0.53   0.57 0.53   48 48   32 31

High  

0.43 0.52 0.025   0.52 0.51   0.47 0.51   40 46   32 35

SE  

0.029 0.031     0.056 0.036   0.035 0.036   3.1 3.1   2.2 2.1

A 43% CP (DM basis) cottonseed meal–based supplement (1.9-cm-diameter pellet) was fed 3 d weekly at rates that averaged either 0.9 or 1.4 kg of DM/steer daily. 2 Low = an initial stocking rate of 29.7 animal-unit-d/ha annually, medium = an initial stocking rate of 33.2 animal-unit-d/ha annually, and high = an initial stocking rate of 39.4 animal-unit-d/ ha annually. 3 Supplementation rate interacted (P ≤ 0.01) with stocking rate. Each level of supplementation was analyzed separately. 4 Quadratic effect (P ≤ 0.01) within supplementation rate. 5 Linear effect (P ≤ 0.09) within supplementation rate. 6 Supplementation rate tended (P ≤ 0.09) to interact with stocking rate. Each level of supplementation was analyzed separately. 1

pasture landscapes (Project No. 6216-21630-007-00D). The author claims no conflicts of interest. The author wishes to express his appreciation to Israel Palascios, Matthew Schneider, and Lonnie Parsons for the field work in this experiment.

Terminology for Grazing Lands and Grazing Animals. Pocahontas Press Inc., Blacksburg, VA.

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ORCIDS Stacey A. Gunter

https:​/​/​orcid​.org/​0000​-0002​-0840​-3555