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Forage production of tropical grasses under extended daylength at subtropical and tropical latitudes
Environmental and Experimental Botany 61 (2007) 18–24
Forage production of tropical grasses under extended daylength at subtropical and tropical latitudes Y.C. Newman a , T.R. Sinclair b,∗ , A.S. Blount b , M.L. Lugo c , E. Valencia c a
b
Texas Agric. Res. Station, 1229 North U.S. Hwy 281, Stephenville, TX 76401, United States University of Florida, 304 Newell Hall P.O. Box 110500, Gainesville, FL 32611-0300, United States c University of Puerto Rico, P.O. Box 1306, Gurabo, PR, United States Received 2 February 2007; accepted 3 February 2007
Abstract Decreased growth of warm-season forage grasses during subtropical winters has been shown to be a response to short-daylength during those months. The objective of this research was to evaluate the influence of short-daylength on growth of forage grasses in low latitude locations. Three grasses (Argentine bahiagrass, Paspalum notatum Fl¨ugge; Common guineagrass, Panicum maximum Jacq.; Tifton-85 bermudagrass, Cynodon spp. Pers.) were grown at three locations (south Florida, Puerto Rico, and St. Croix) under extended (15 h) and natural daylength during 2 years. Plots were harvested at 5-week (October–February) and 4-week (March–April) intervals to measure dry matter (DM) yield and nutritive value of the herbage. Daylength effects were found on DM harvested for all grasses in south Florida (P < 0.01). At that location, grasses exposed to increased daylength averaged a 44% increase in DM harvested compared to those grown under natural daylength. Daylength effects were consistent for bahiagrass at the other two locations as well. However, guineagrass and bermudagrass did not respond to extended daylength in Puerto Rico and St. Croix. These results showed that bahiagrass is capable of producing additional forage during the cool season at all locations tested if sensitivity to photoperiod is genetically removed or attenuated. © 2007 Elsevier B.V. All rights reserved. Keywords: Paspalum notatum; Panicum maximum; Cynodon spp.; Daylength; Photoperiod; Forage production; Forage nutritive value
1. Introduction Growth and plant development of tropical forages is variable throughout the year and affected by several environmental factors. Cool temperatures, low levels of solar radiation and low rainfall are among the environmental constraints that have been associated with lower production during the winter months. Simulations of forage growth in tropical locations (Sinclair et al., 2005) indicated that daylength may play a major role in the observed decreases in forage production during the shortdaylength period of the year. Previously, it was demonstrated that short-daylength inhibited the growth under field conditions of bahiagrass and bermudagrass in central Florida (Sinclair et al., 2001, 2003). The fraction of the total annual dry matter (DM) production of
Abbreviations: CP, crude protein; IVDOM, in vitro digestible organic matter; OM, organic matter ∗ Corresponding author. Tel.: +1 352 392 6180; fax: +1 352 392 6139. E-mail address: [email protected] (T.R. Sinclair). 0098-8472/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.envexpbot.2007.02.005
bahiagrass in north-central Florida under natural photoperiod was zero during the winter months, 30% during the late fall, and 15% in the early spring (Newman et al., 2001). Low production by bahiagrass in the winter months is particularly restrictive for cattle production in the southeastern U.S. (Burton, 1967). Reduced forage production during the short-daylength period of the year has also been reported in low-latitude areas. In Puerto Rico, Ramos-Santana and Rodriguez-Arroyo (1991) reported 22% lower winter-season yield of guineagrass cultivars when compared to production during long days. However, it was not determined whether the loss of production was a result of altered plant growth due to the short-daylengths, or a response to other environmental characteristics such as low temperature or water deficits. The objective of this study was to evaluate under field conditions the influence of artificially extended daylength on forage production of tropical grasses in subtropical and tropical regions. Parallel experiments were undertaken at three locations (south Florida, Puerto Rico, and St. Croix) by placing lights in the field to impose a treatment of extended daylength (15 h) throughout the year. In addition to monitoring growth, changes in nutritive
Y.C. Newman et al. / Environmental and Experimental Botany 61 (2007) 18–24
value of the forage as a result of the daylength treatment were measured. 2. Materials and methods 2.1. Location and site characteristics The experiment was conducted concurrently at three locations spanning latitudes from 17 to 26◦ N. These locations fall within the area of the northern hemisphere where there is over 3000 h year−1 of sunshine. This area typically has less cloud cover than lower latitudes (0–15◦ N). The northern-most site was south Florida, at the Southwest Research Center, University of Florida (26◦ 27 44 N, 81◦ 26 25 W at 25 m above sea level) near Immokalee, Florida. The other two sites were University of Puerto Rico at Gurabo, PR (18◦ 15 30 N, 65◦ 59 32 W, at 160 m above sea level) and a site near Frederiksted, St. Croix, U.S. Virgin Islands (17◦ 45 00 N, 68◦ 48 00 W at 45 m above sea level). Minimum daylength at the winter solstice was 10 h 29 min for Immokalee, Fl; 11 h 4 min for Gurabo, PR; 11 h 2 min for St. Croix. Soil type in South Florida was Immokalee fine sand (sandy, siliceous, hyperthermic Arenic Haplaquod). This soil is poorly drained but characterized by sand in excess of 95% in the top 1 m. Soil in Puerto Rico was of the Mabi series (fine montmorillonitic, isohyperthermic Vertic Eutropepts). This soil is rather poorly drained with medium to slow runoff. Soil type for St. Croix was mildly alkaline Fredensborg clay (fine carbonatic, isohyperthermic, Typic Rendolls). These are well drained soils with medium runoff. 2.2. Field procedures and laboratory analysis Seeds of ‘Argentine’ bahiagrass (Paspalum notatum Flugge), a local guineagrass (Panicum maximum Jacq.) ecotype collected near Frederiksted, St. Croix, and stolons from ‘Tifton 85’ bermudagrass (Cynodon spp.; obtained as certified Tifton 85 foundation material from the Coastal Plain Experiment Station, Tifton, Georgia) were sown into 50 mm × 50 mm containers of sterile Metro Mix 200TM potting soil (Scotts-Sierra, Marysville, OH 43041) on 19 February 2002 at the North Florida Research and Education Center in Quincy, Florida. Plants were grown in the greenhouse at 28 ◦ C, fertilized monthly with 0.1 g plant−1 Osmocote 14–14–14 (N–P2 O5 –K2 O) (ScottsSierra, Marysville, OH 43041), and watered daily. On 22 May 2002, seedlings were washed free of soil, and inspected by the Florida Division of Plant Industry and shipped to Puerto Rico and St. Croix. Seedlings were transplanted to the field at all locations during the week of 11 June 2002. At each of the three locations, six replicate plots of each grass were established in each of the 2 daylength treatments (natural daylength and daylength extended to 15 h). The individual plots for each species were 1.21 m2 (1.1 m × 1.1 m) and harvests were made from the inner 1 m2 of each plot. Fertilization of plots at each site throughout the experiment was adapted to the soil fertility at that location. Fertilizer applied at South Florida totaled 228 kg N, 14 kg P, and
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93 kg K ha−1 year−1 , where applications (19 kg N, 1.2 kg P, and 7.5 kg K ha−1 ) were conducted monthly after each harvest. At the Puerto Rico site, fertilizer applied totaled 500 kg N, 70 kg P, and 280 kg K ha−1 year−1 and applications (83 kg N, 12 kg P, and 47 kg K ha−1 ) were made every 2 months. At the St. Croix site, 56 kg N ha−1 year−1 were applied as a one time application in February 2003 and January 2004. Plots at all locations were irrigated as needed to avoid the development of water-deficit stress. Harvests at 4- or 5-week intervals were coordinated among the three locations so that they were made during the same week. Plants were harvested during the weeks of 7 October, 12 November, 16 December, 20 January, 24 February, 24 March and 21 April (2002–2003) and 6 October, 10 November, 15 December, 18 January, 23 February, 22 March and 19 April (2003–2004). At each harvest date, herbage DM accumulation for each plot was measured by clipping the plots manually. The stubble height was 10 cm for sod-type grasses (bahiagrass and bermudagrass), and 30 cm for the bunch-type guineagrass. All the harvested material was dried for at least 48 h at 55 ◦ C and then weighed. Samples were ground to pass through a 1mm mesh screen in a Wiley mill. Ground forage samples from two field replicates harvested on 11 November 2002 and 11 November 2003 were analyzed for total N concentration and in vitro digestible organic matter (IVDOM). Samples from all locations were sent to the Forage Nutrition Laboratory, University of Florida, Gainesville, FL where analyses were done on all samples. A 1-g aliquot from each sample, corresponding to a replicate and treatment combination, was used for determining absolute DM (drying at 105 ◦ C for 15 h; AOAC, 1990). Total organic matter (OM) was determined by ashing for 15 h at 550 ◦ C (AOAC, 1990). The two-stage technique of Moore and Mott (1974) was used to determine IVDOM. Total N was determined using a micro-Kjeldahl method, a modification of the aluminum block digestion technique described by Gallaher et al. (1975). Crude protein was calculated by multiplying nitrogen concentration by 6.25 (AOAC, 1990). 2.3. Daylength treatments Extended daylength was achieved using four quartz-halogen lamps (1500 W, 240 V). Two lamps were mounted at 2 m above the soil on posts at each of the short sides of the main plot (3.6 m × 7.2 m). The lamp fixtures were positioned to shine light across the main plot treatment from the two opposing sides and the minimum photosynthetic radiation measured at any individual plot was 10.3 (Florida), 7.2 (Puerto Rico) and 7.3 mol m−2 s−1 (St. Croix); all of which were above the saturation threshold required for subtropical grasses of 6 mol m−2 s−1 (Sinclair et al., 2004). The overall photosynthetic radiation mean maintained during extended daylength was 20 (south Florida), 19 (Puerto Rico), and 22 mol m−2 s−1 (St. Croix). Timers were used to control when the lamps were on. The lamps were activated 30 min before sunset and turned off when the total photoperiod had reached 15 h (counted from sunrise). Thus, timers were adjusted as the season progressed to achieve the desired photoperiod extension of 15 h.
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2.4. Meteorological data Meteorological data corresponded to daily observations obtained from weather stations at the different locations. The south Florida data were retrieved through the Florida automated weather network (FAWN) system. Weather variations through the annual cycle were examined through 7-days summaries using SAS basic programming (SAS Institute Inc., 1996). Mean values were calculated for minimum and maximum temperatures, and 7-days sums were computed for precipitation. 2.5. Statistical analyses Light-control data and nutritive value variables were analyzed using a two-group comparison methodology (Littell et al., 2002) where the effect of daylength was considered fixed. Herbage DM harvested was analyzed as a three-way factorial experiment with PC SAS version 9.1.2 (SAS Institute Inc., 2004) through PROC MIXED (SAS Institute Inc., 1996). In the model, effects of daylength, location, species, and their interactions were considered the sources of variation. Due to the presence of interactions, data are presented by location and species. Interactions were explored using the SLICE option technique to maintain the power and efficiency of the model. All means reported in the text are least squares means. The seasonal distribution of herbage DM harvested was analyzed using a two group comparison (natural versus extended light) and the significance of the treatment is expressed for each harvest by location and species. 3. Results 3.1. Weather data Minimum and maximum temperatures are presented for the three locations for a 2-year period starting July 2002 (Fig. 1). Based on 7-day temperature averages, the weather observed for the locations corresponds to a subtropical climate for south Florida, tropical climate for Puerto Rico, and maritime-tropical climate for St. Croix. Minimum and maximum temperatures in south Florida were the most variable among the three locations. In south Florida, daily minimum and maximum temperatures differed by about 20 ◦ C in the short-day months while the difference decreased to about 15 ◦ C during the long-day months. Based on 7-day averages, minimum temperatures were never below 3 ◦ C (2002–2003 short-days season) or 5 ◦ C (2003–2004), and never above 20 ◦ C for both periods. However, three freeze events were recorded in the first season (18, 19, and 24 January 2003), and two in the second (21 December 2003 and 24 January 2004). Temperatures in Puerto Rico and St. Croix, as expected, did not fluctuate as much as those of south Florida. However, the annual temperature profile did highlight differences between these two tropical locations. In Puerto Rico, maximum temperatures through the November to April season ranged from 27 to 33 ◦ C and minimum temperatures ranged from 16 to 22 ◦ C. Consequently, the difference between maximum and minimum
Fig. 1. Maximum and minimum ambient air temperatures at 7-day intervals for experimental sites in South Florida, Puerto Rico, and St. Croix. Values extracted from daily observations from January 2002 to July 2004.
temperature was about 10–11 ◦ C. In St. Croix, the maximum temperature was nearly the same as in Puerto Rico but the minimum temperature was consistently higher. Hence, the difference between maximum and minimum temperatures in St. Croix was only about 6–7 ◦ C. The small daily fluctuation in temperature is typical of the maritime-tropical climate prevailing at St. Croix. South Florida had the largest daily fluctuation due to the continental climate that influences the weather at this location (Henry et al., 1994). 3.2. DM harvested 3.2.1. Light and species effects There was a species × daylength × location interaction (P = 0.06) for DM harvested, therefore, data were analyzed by location. When analyzed by location, there were species (P ≤ 0.01) and daylength (P ≤ 0.01) effects at South Florida, a daylength × species interaction for Puerto Rico (P = 0.01), but no effect for St. Croix (P = 0.45). The analysis of simple effects for St. Croix gave some indication that daylength affected bahiagrass growth (P = 0.18; Table 1); therefore this trend of daylength × species interaction, consistent with results at the other two locations, is considered. Averaged across daylength treatments, DM harvested in south Florida during short-daylength months of October through April was greatest for guineagrass, intermediate for bermuda-
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Table 1 Light and species means for total annual DM harvested at South Florida, and light × specie interaction for Puerto Rico and St. Croix Species
South Florida
Puerto Rico
St. Croix
Natural (Mg ha− )
Extended (Mg ha− )
P-value
Natural (Mg ha− )
Extended (Mg ha− )
P-value
Natural (Mg ha− )
Extended (Mg ha− )
P-value
Bahiagrass Guineagrass Bermudagrass
0.14 1.31 0.59
0.49 1.50 0.93
<0.01 <0.01 <0.01
0.85 4.10 2.23
1.64 4.22 2.15
<0.01 0.52 0.62
0.56 3.19 1.26
0.80 3.23 1.19
0.18 0.80 0.66
Mean
0.68b
0.98a‡
0.04
Data are means averaged across 14 harvests (October–April in 2003 and 2004) and 6 replicates (n = 84). ‡ Daylength means within a row not followed by the same letter are different (P < 0.01).
grass, and lowest for bahiagrass (1.4, 0.76, and 0.32 Mg ha−1 , respectively). Daylength effects (Table 1) were present at this location for all three grasses (P < 0.01). Averaged across grasses, exposure to extended daylength resulted in an average 44% increase in DM harvested during the short-daylength months from 0.68 Mg ha−1 for natural daylength to 0.98 Mg ha−1 for the extended photoperiod. Specifically, DM harvested for bahiagrass increased (P < 0.01; Table 1) from 0.14 Mg ha−1 under natural daylength to 0.49 Mg ha−1 under extended daylength, which amounted to an increment of 3.5 times. Harvested DM for guineagrass increased 15%
from 1.31 Mg ha−1 for the natural daylength treatment to 1.50 Mg ha−1 for the extended daylength treatment. For bermudagrass, the increase was 58% from 0.59 Mg ha−1 under natural daylength compared to 0.93 Mg ha−1 when grown under extended photoperiod. At Puerto Rico, only bahiagrass showed a significant increase in DM when exposed to extended daylength (P < 0.01; Table 1). Bahiagrass DM harvested during October through April under extended daylength was 92% greater than for natural daylength (1.64 Mg ha−1 versus 0.85 Mg ha−1 ). Daylength had no influence on guineagrass (P = 0.52) or bermudagrass (P = 0.62)
Fig. 2. Bahiagrass dry matter (DM) harvested means under extended- and natural-daylength treatments for the locations of south Florida, Puerto Rico and St. Croix. The symbols represent the significance of the treatments at each harvest for the winter season 2002–2003 and 2003–2004 (NS, not significant, † P < 0.1; * P < 0.05; ** P < 0.01).
Fig. 3. Guineagrass dry matter (DM) harvested means under extended- and natural-daylength treatments for the locations of south Florida, Puerto Rico and St. Croix. The symbols represent the significance of the treatments at each harvest for the winter season 2002–2003 and 2003–2004 (NS, not significant, † P < 0.1; * P < 0.05; ** P < 0.01).
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DM harvested. Harvested DM of guineagrass was 4.10 and 4.22 Mg ha−1 and 2.23 and 2.15 Mg ha−1 for bermudagrass for natural and extended daylength, respectively. In St. Croix, when simple effects were explored, species × daylength effects approached significance (Table 1). Bahiagrass DM increased 43% from 0.56 Mg ha−1 when grown under natural daylength to 0.80 Mg ha−1 under extended (P = 0.18); but similar to Puerto Rico, guineagrass and bermudagrass did not show any response to daylength at this location (3.19 Mg ha−1 versus 3.23 Mg ha−1 and 1.26 Mg ha−1 versus 1.19 Mg ha−1 , respectively, for guineagrass and bahiagrass under natural and extended daylength).
February and March 2004. Harvest data in St. Croix presented greater experimental errors compared to the other locations that did not allow detection of statistical differences. Those errors were associated with episodes of intermittent operational problems with the forced-air oven dryer. Increased DM of guineagrass with extended daylength (Fig. 3) occurred only in south Florida for the November and December 2002 harvests and in St. Croix for the January 2003 harvest. Bermudagrass had increased DM with extended daylength (Fig. 4) only in south Florida from November 2002 to March 2003 but the effect was not consistent in the harvests of October 2003–April 2004.
3.2.2. Light effects and seasonal distribution of DM harvested Daylength effects on DM harvested for bahiagrass (Fig. 2) were reflected at each harvest from November 2002 to April 2003, and December 2003 to April 2004. Daylength effects on DM harvested in Puerto Rico were also observed at all harvests from October 2002 to April 2003, and November 2003 to April 2004. Dry matter harvested at St. Croix showed daylength effects from October 2002 to March 2003, but the effects were not present during most of the short-daylength months of the 2nd year (October 2003 to January 2004) and were observed only in
3.3. Herbage nutritive value
Fig. 4. Bermudagrass dry matter (DM) harvested means under extended- and natural-daylength treatments for the locations of south Florida, Puerto Rico and St. Croix. The symbols represent the significance of the treatments at each harvest for the winter season 2002–2003 and 2003–2004 (NS, not significant, † P < 0.1; * P < 0.05; ** P < 0.01).
3.3.1. Daylength effects for CP and IVDOM There were strong location effects (P = 0.02) and interactions with location (P < 0.01) for CP and IVDOM, respectively; there-
Fig. 5. Crude protein (CP) concentration for bahiagrass, guineagrass and bermudagrass herbage harvested at South Florida (A), Puerto Rico (B), and St. Croix (C). Data are means averaged across two harvests (11 November 2002, and 11 November 2003) and two replicates (n = 4). The symbols represent the significance of the treatments at each harvest for the winter season 2002–2003 and 2003–2004 (NS, not significant, † P < 0.1; * P < 0.05).
Y.C. Newman et al. / Environmental and Experimental Botany 61 (2007) 18–24
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In contrast to bahiagrass, there was no effect of daylength on herbage CP of guineagrass or bermudagrass. This lack of effect was similar across all locations. The averages for natural versus extended daylength were 119 g kg−1 versus 131 g kg−1 for guineagrass (P = 0.24) and 136 g kg−1 versus 130 g kg−1 for bermudagrass (P = 0.59), respectively. Comparison of IVDOM when analyzed by location, showed a response to daylength at St. Croix (Fig. 6C). There was a significant increase in IVDOM of guineagrass in response to the extended daylength as compared to the natural daylength (608 g kg−1 versus 481 g kg−1 , respectively). In contrast, the IVDOM of bahiagrass grown under extended daylength in St. Croix was lower relative to the natural daylength treatment (463 g kg−1 versus 538 g kg−1 , respectively, P = 0.11). At south Florida and Puerto Rico there was no daylength effect on IVDOM for any species. 4. Discussion and conclusions
Fig. 6. In vitro digestible organic matter (IVDOM) for bahiagrass, guineagrass, and bermudagrass herbage harvested at South Florida (A), Puerto Rico (B), and St. Croix (C). Data are means averaged across two harvests (11 November 2002, and 11 November 2003) and two replicates (n = 4). The symbols represent the significance of the treatments at each harvest for the winter season 2002–2003 and 2003–2004 (NS, not significant, † P < 0.1).
fore, the data are presented by location (Figs. 5 and 6). Herbage CP concentration had a daylength × species interaction effect (P < 0.01). Interaction occurred because herbage CP concentrations of bahiagrass were significantly (P < 0.01) higher for plants exposed to natural daylength than for plants exposed to extended daylength. The herbage CP concentrations of bahiagrass for natural versus extended daylength treatments were 136 g kg−1 versus 115 g kg−1 at south Florida, 140 g kg−1 versus 104 g kg−1 at Puerto Rico, and 157 g kg−1 versus 109 g kg−1 at St. Croix. Across locations, the difference between daylength treatments amounted to an average decrease of 35 g CP kg−1 for the extended daylength treatment (109 g kg−1 ) as compared to the natural daylength treatment (144 g kg−1 ). Lower CP values of bahiagrass under extended daylength may correspond to a dilution effect. Given the same CP content in the tissue, the DM harvested was lower for the natural daylength but was greater under extended photoperiod.
During short days, forage DM harvested of bahiagrass, guineagrass, and bermudagrass varied depending on the experimental location. Bahiagrass consistently had greater DM harvested across all three locations in response to extended daylength during October through April. The positive response in St. Croix with a lower fertilizer application proved that if photoperiod had an influence even minimum fertilizer should allow a positive response as was the case in this location. The findings from these three locations corroborate the considerable sensitivity of bahiagrass to photoperiod that has been reported in subtropical areas (Marousky et al., 1991; Newman et al., 2004; Sinclair et al., 2001, 2003). In contrast to bahiagrass, guineagrass and bermudagrass had significant increases in DM harvested under extended daylength only at the subtropical site (south Florida). The increase in DM with extended daylength for guineagrass and bermudagrass that was present in south Florida was not observed at lower, tropical latitudes (Puerto Rico and St. Croix; P > 0.52). The failure to alleviate in guineagrass and bermudagrass at least part of the decline in growth during the short-daylength period with extended daylength implies that there are other factors involved in growth regulation of these two grasses. In terms of nutritive value, CP of bahiagrass was consistently lower at all locations under the extended daylength treatment. Lower CP of bahiagrass growing under extended daylength compared to those growing under natural daylength plots is at least partially accounted for by a dilution effect due to the greater DM harvested under extended light. These results corroborate previous results with bahiagrass at higher latitude (Sinclair et al., 2003). The CP in either case was not marginal and was above the minimum critical levels (60–80 g kg−1 ) required for livestock production and proper rumen-bacteria functioning (Humphreys, 1991; Van Soest, 1994). In vitro digestible organic matter was only affected by extended daylength at St. Croix. The onset of plant reproduction has been recognized as a crucial factor in C4 grass herbage quality decline (Coleman et al., 2004). Given the potential of plant development stage to affect nutritive value and yield, a detailed analysis of the plant development pattern in
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response to daylength seems warranted to fully understand the observed responses. The implications of our results are that bahiagrass is capable of producing additional forage in the winter season if sensitivity to photoperiod is removed or attenuated, but this does occur at the expense of herbage CP concentration. Thus, there is room for increasing the production of bahiagrass and the challenge lies in selecting and developing photoperiod insensitive plants of this species. Acknowledgments The authors thank are grateful for the participation and field assistance of Mr. Hector Torres and Mr. Stuart Weiss (research associates in Puerto Rico and St. Croix Research Stations, respectively), and the field crew at Immokalee Research Station for their cooperation and sustained effort in maintaining and harvesting the plots which were key aspects in this long-term experiment. References AOAC, 1990. Official Methods of Analysis, 15th ed. AOAC, Arlington, VA. Burton, G.W., 1967. A search for the origin of Pensacola bahia grass. Econ. Bot. 21, 379–382. Coleman, S.W., Moore, J.E., Wilson, J.R., 2004. Quality and utilization. In: Moser, L.E., et, al. (Eds.), Warm-season C4 Grasses. ASA/CSSA/SSA, Madison, WI, pp. 267–308. Gallaher, R.N., Weldon, C.O., Futral, J.G., 1975. An aluminum block digester for plant and soil analysis. Soil Sci. Soc. Am. Proc. 39, 803–806. Henry, J.A., Portier, K.M., Coyne, J., 1994. The Climate and Weather of Florida. Pineapple Press Inc., Sarasota, FL.
Humphreys, L.R., 1991. Tropical Pasture Utilization. Cambridge University Press, Cambridge. Littell, R.C., Freund, R.J., Spector, P.C., 2002. SAS System for Linear Models. SAS Inst., Cary, NC. Marousky, F.J., Ploetz, R.C., Clayton, D.C., Chambliss, C.G., 1991. Flowering response of Pensacola and Tifton 9 bahiagrasses grown at different latitudes. Soil Crop Sci. Soc. Fla. Proc. 50, 65–69. Moore, J.E., Mott, G.O., 1974. Recovery of residual organic matter from in vitro digestion of forages. J. Dairy Sci. 57, 1258–1259. Newman, Y.C., Sollenberger, L.E., Boote, K.J., Allen Jr., L.H., Littell, R.C., 2001. Carbon dioxide and temperature effects on forage dry matter production. Crop Sci. 41, 399–406. Newman, Y.C., Sinclair, T.R., Blount, A.R., Lugo, M.L., Valencia, E., 2004. Flowering of tropical forages exposed to extended photoperiod. In: 2004 Agronomy abstracts. ASA, Madison, WI. Ramos-Santana, R., Rodriguez-Arroyo, J.E., 1991. Seasonal production of 11 Panicum maximum cultivars harvested at a 45-day interval. J. Agric. Univ. P. R. 75, 61–66. SAS Institute Inc., 2004. SAS/STAT 9.1 User’s guide. SAS Institute Inc., Cary, NC. SAS Institute Inc., 1996. SAS/STAT Software: Changes and Enhancements Through Release 6.11. SAS Institute Inc., Cary, NC. Sinclair, T.R., Mislevy, P., Ray, J.D., 2001. Short photoperiod inhibits winter growth of subtropical grasses. Plantarum 213, 488–491. Sinclair, T.R., Ray, J.D., Mislevy, P., Premazzi, L.M., 2003. Growth of subtropical forage grasses under extended photoperiod during short-daylength months. Crop Sci. 43, 618–623. Sinclair, T.R., Ray, J.D., Premazzi, L.M., Mislevy, P., 2004. Photosynthetic photon flux density influences grass responses to extended photoperiod. Environ. Exp. Bot. 51, 69–74. Sinclair, T.R., Newman, Y.C., Lugo, M.L., Valencia, E., Blount, A.R., 2005. Potential for year-round forage production in Puerto Rico and St. Croix. J. Agric. Univ. P. R. 89, 133–148. Van Soest, P.J., 1994. Nitrogen metabolism. In: Van Soest, J.P. (Ed.), Nutritional Ecology of the Ruminant. Cornell University Press, Ithaca, New York.
Forage production of tropical grasses under extended daylength at subtropical and tropical latitudes Y.C. Newman a , T.R. Sinclair b,∗ , A.S. Blount b , M.L. Lugo c , E. Valencia c a
b
Texas Agric. Res. Station, 1229 North U.S. Hwy 281, Stephenville, TX 76401, United States University of Florida, 304 Newell Hall P.O. Box 110500, Gainesville, FL 32611-0300, United States c University of Puerto Rico, P.O. Box 1306, Gurabo, PR, United States Received 2 February 2007; accepted 3 February 2007
Abstract Decreased growth of warm-season forage grasses during subtropical winters has been shown to be a response to short-daylength during those months. The objective of this research was to evaluate the influence of short-daylength on growth of forage grasses in low latitude locations. Three grasses (Argentine bahiagrass, Paspalum notatum Fl¨ugge; Common guineagrass, Panicum maximum Jacq.; Tifton-85 bermudagrass, Cynodon spp. Pers.) were grown at three locations (south Florida, Puerto Rico, and St. Croix) under extended (15 h) and natural daylength during 2 years. Plots were harvested at 5-week (October–February) and 4-week (March–April) intervals to measure dry matter (DM) yield and nutritive value of the herbage. Daylength effects were found on DM harvested for all grasses in south Florida (P < 0.01). At that location, grasses exposed to increased daylength averaged a 44% increase in DM harvested compared to those grown under natural daylength. Daylength effects were consistent for bahiagrass at the other two locations as well. However, guineagrass and bermudagrass did not respond to extended daylength in Puerto Rico and St. Croix. These results showed that bahiagrass is capable of producing additional forage during the cool season at all locations tested if sensitivity to photoperiod is genetically removed or attenuated. © 2007 Elsevier B.V. All rights reserved. Keywords: Paspalum notatum; Panicum maximum; Cynodon spp.; Daylength; Photoperiod; Forage production; Forage nutritive value
1. Introduction Growth and plant development of tropical forages is variable throughout the year and affected by several environmental factors. Cool temperatures, low levels of solar radiation and low rainfall are among the environmental constraints that have been associated with lower production during the winter months. Simulations of forage growth in tropical locations (Sinclair et al., 2005) indicated that daylength may play a major role in the observed decreases in forage production during the shortdaylength period of the year. Previously, it was demonstrated that short-daylength inhibited the growth under field conditions of bahiagrass and bermudagrass in central Florida (Sinclair et al., 2001, 2003). The fraction of the total annual dry matter (DM) production of
Abbreviations: CP, crude protein; IVDOM, in vitro digestible organic matter; OM, organic matter ∗ Corresponding author. Tel.: +1 352 392 6180; fax: +1 352 392 6139. E-mail address: [email protected] (T.R. Sinclair). 0098-8472/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.envexpbot.2007.02.005
bahiagrass in north-central Florida under natural photoperiod was zero during the winter months, 30% during the late fall, and 15% in the early spring (Newman et al., 2001). Low production by bahiagrass in the winter months is particularly restrictive for cattle production in the southeastern U.S. (Burton, 1967). Reduced forage production during the short-daylength period of the year has also been reported in low-latitude areas. In Puerto Rico, Ramos-Santana and Rodriguez-Arroyo (1991) reported 22% lower winter-season yield of guineagrass cultivars when compared to production during long days. However, it was not determined whether the loss of production was a result of altered plant growth due to the short-daylengths, or a response to other environmental characteristics such as low temperature or water deficits. The objective of this study was to evaluate under field conditions the influence of artificially extended daylength on forage production of tropical grasses in subtropical and tropical regions. Parallel experiments were undertaken at three locations (south Florida, Puerto Rico, and St. Croix) by placing lights in the field to impose a treatment of extended daylength (15 h) throughout the year. In addition to monitoring growth, changes in nutritive
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value of the forage as a result of the daylength treatment were measured. 2. Materials and methods 2.1. Location and site characteristics The experiment was conducted concurrently at three locations spanning latitudes from 17 to 26◦ N. These locations fall within the area of the northern hemisphere where there is over 3000 h year−1 of sunshine. This area typically has less cloud cover than lower latitudes (0–15◦ N). The northern-most site was south Florida, at the Southwest Research Center, University of Florida (26◦ 27 44 N, 81◦ 26 25 W at 25 m above sea level) near Immokalee, Florida. The other two sites were University of Puerto Rico at Gurabo, PR (18◦ 15 30 N, 65◦ 59 32 W, at 160 m above sea level) and a site near Frederiksted, St. Croix, U.S. Virgin Islands (17◦ 45 00 N, 68◦ 48 00 W at 45 m above sea level). Minimum daylength at the winter solstice was 10 h 29 min for Immokalee, Fl; 11 h 4 min for Gurabo, PR; 11 h 2 min for St. Croix. Soil type in South Florida was Immokalee fine sand (sandy, siliceous, hyperthermic Arenic Haplaquod). This soil is poorly drained but characterized by sand in excess of 95% in the top 1 m. Soil in Puerto Rico was of the Mabi series (fine montmorillonitic, isohyperthermic Vertic Eutropepts). This soil is rather poorly drained with medium to slow runoff. Soil type for St. Croix was mildly alkaline Fredensborg clay (fine carbonatic, isohyperthermic, Typic Rendolls). These are well drained soils with medium runoff. 2.2. Field procedures and laboratory analysis Seeds of ‘Argentine’ bahiagrass (Paspalum notatum Flugge), a local guineagrass (Panicum maximum Jacq.) ecotype collected near Frederiksted, St. Croix, and stolons from ‘Tifton 85’ bermudagrass (Cynodon spp.; obtained as certified Tifton 85 foundation material from the Coastal Plain Experiment Station, Tifton, Georgia) were sown into 50 mm × 50 mm containers of sterile Metro Mix 200TM potting soil (Scotts-Sierra, Marysville, OH 43041) on 19 February 2002 at the North Florida Research and Education Center in Quincy, Florida. Plants were grown in the greenhouse at 28 ◦ C, fertilized monthly with 0.1 g plant−1 Osmocote 14–14–14 (N–P2 O5 –K2 O) (ScottsSierra, Marysville, OH 43041), and watered daily. On 22 May 2002, seedlings were washed free of soil, and inspected by the Florida Division of Plant Industry and shipped to Puerto Rico and St. Croix. Seedlings were transplanted to the field at all locations during the week of 11 June 2002. At each of the three locations, six replicate plots of each grass were established in each of the 2 daylength treatments (natural daylength and daylength extended to 15 h). The individual plots for each species were 1.21 m2 (1.1 m × 1.1 m) and harvests were made from the inner 1 m2 of each plot. Fertilization of plots at each site throughout the experiment was adapted to the soil fertility at that location. Fertilizer applied at South Florida totaled 228 kg N, 14 kg P, and
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93 kg K ha−1 year−1 , where applications (19 kg N, 1.2 kg P, and 7.5 kg K ha−1 ) were conducted monthly after each harvest. At the Puerto Rico site, fertilizer applied totaled 500 kg N, 70 kg P, and 280 kg K ha−1 year−1 and applications (83 kg N, 12 kg P, and 47 kg K ha−1 ) were made every 2 months. At the St. Croix site, 56 kg N ha−1 year−1 were applied as a one time application in February 2003 and January 2004. Plots at all locations were irrigated as needed to avoid the development of water-deficit stress. Harvests at 4- or 5-week intervals were coordinated among the three locations so that they were made during the same week. Plants were harvested during the weeks of 7 October, 12 November, 16 December, 20 January, 24 February, 24 March and 21 April (2002–2003) and 6 October, 10 November, 15 December, 18 January, 23 February, 22 March and 19 April (2003–2004). At each harvest date, herbage DM accumulation for each plot was measured by clipping the plots manually. The stubble height was 10 cm for sod-type grasses (bahiagrass and bermudagrass), and 30 cm for the bunch-type guineagrass. All the harvested material was dried for at least 48 h at 55 ◦ C and then weighed. Samples were ground to pass through a 1mm mesh screen in a Wiley mill. Ground forage samples from two field replicates harvested on 11 November 2002 and 11 November 2003 were analyzed for total N concentration and in vitro digestible organic matter (IVDOM). Samples from all locations were sent to the Forage Nutrition Laboratory, University of Florida, Gainesville, FL where analyses were done on all samples. A 1-g aliquot from each sample, corresponding to a replicate and treatment combination, was used for determining absolute DM (drying at 105 ◦ C for 15 h; AOAC, 1990). Total organic matter (OM) was determined by ashing for 15 h at 550 ◦ C (AOAC, 1990). The two-stage technique of Moore and Mott (1974) was used to determine IVDOM. Total N was determined using a micro-Kjeldahl method, a modification of the aluminum block digestion technique described by Gallaher et al. (1975). Crude protein was calculated by multiplying nitrogen concentration by 6.25 (AOAC, 1990). 2.3. Daylength treatments Extended daylength was achieved using four quartz-halogen lamps (1500 W, 240 V). Two lamps were mounted at 2 m above the soil on posts at each of the short sides of the main plot (3.6 m × 7.2 m). The lamp fixtures were positioned to shine light across the main plot treatment from the two opposing sides and the minimum photosynthetic radiation measured at any individual plot was 10.3 (Florida), 7.2 (Puerto Rico) and 7.3 mol m−2 s−1 (St. Croix); all of which were above the saturation threshold required for subtropical grasses of 6 mol m−2 s−1 (Sinclair et al., 2004). The overall photosynthetic radiation mean maintained during extended daylength was 20 (south Florida), 19 (Puerto Rico), and 22 mol m−2 s−1 (St. Croix). Timers were used to control when the lamps were on. The lamps were activated 30 min before sunset and turned off when the total photoperiod had reached 15 h (counted from sunrise). Thus, timers were adjusted as the season progressed to achieve the desired photoperiod extension of 15 h.
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2.4. Meteorological data Meteorological data corresponded to daily observations obtained from weather stations at the different locations. The south Florida data were retrieved through the Florida automated weather network (FAWN) system. Weather variations through the annual cycle were examined through 7-days summaries using SAS basic programming (SAS Institute Inc., 1996). Mean values were calculated for minimum and maximum temperatures, and 7-days sums were computed for precipitation. 2.5. Statistical analyses Light-control data and nutritive value variables were analyzed using a two-group comparison methodology (Littell et al., 2002) where the effect of daylength was considered fixed. Herbage DM harvested was analyzed as a three-way factorial experiment with PC SAS version 9.1.2 (SAS Institute Inc., 2004) through PROC MIXED (SAS Institute Inc., 1996). In the model, effects of daylength, location, species, and their interactions were considered the sources of variation. Due to the presence of interactions, data are presented by location and species. Interactions were explored using the SLICE option technique to maintain the power and efficiency of the model. All means reported in the text are least squares means. The seasonal distribution of herbage DM harvested was analyzed using a two group comparison (natural versus extended light) and the significance of the treatment is expressed for each harvest by location and species. 3. Results 3.1. Weather data Minimum and maximum temperatures are presented for the three locations for a 2-year period starting July 2002 (Fig. 1). Based on 7-day temperature averages, the weather observed for the locations corresponds to a subtropical climate for south Florida, tropical climate for Puerto Rico, and maritime-tropical climate for St. Croix. Minimum and maximum temperatures in south Florida were the most variable among the three locations. In south Florida, daily minimum and maximum temperatures differed by about 20 ◦ C in the short-day months while the difference decreased to about 15 ◦ C during the long-day months. Based on 7-day averages, minimum temperatures were never below 3 ◦ C (2002–2003 short-days season) or 5 ◦ C (2003–2004), and never above 20 ◦ C for both periods. However, three freeze events were recorded in the first season (18, 19, and 24 January 2003), and two in the second (21 December 2003 and 24 January 2004). Temperatures in Puerto Rico and St. Croix, as expected, did not fluctuate as much as those of south Florida. However, the annual temperature profile did highlight differences between these two tropical locations. In Puerto Rico, maximum temperatures through the November to April season ranged from 27 to 33 ◦ C and minimum temperatures ranged from 16 to 22 ◦ C. Consequently, the difference between maximum and minimum
Fig. 1. Maximum and minimum ambient air temperatures at 7-day intervals for experimental sites in South Florida, Puerto Rico, and St. Croix. Values extracted from daily observations from January 2002 to July 2004.
temperature was about 10–11 ◦ C. In St. Croix, the maximum temperature was nearly the same as in Puerto Rico but the minimum temperature was consistently higher. Hence, the difference between maximum and minimum temperatures in St. Croix was only about 6–7 ◦ C. The small daily fluctuation in temperature is typical of the maritime-tropical climate prevailing at St. Croix. South Florida had the largest daily fluctuation due to the continental climate that influences the weather at this location (Henry et al., 1994). 3.2. DM harvested 3.2.1. Light and species effects There was a species × daylength × location interaction (P = 0.06) for DM harvested, therefore, data were analyzed by location. When analyzed by location, there were species (P ≤ 0.01) and daylength (P ≤ 0.01) effects at South Florida, a daylength × species interaction for Puerto Rico (P = 0.01), but no effect for St. Croix (P = 0.45). The analysis of simple effects for St. Croix gave some indication that daylength affected bahiagrass growth (P = 0.18; Table 1); therefore this trend of daylength × species interaction, consistent with results at the other two locations, is considered. Averaged across daylength treatments, DM harvested in south Florida during short-daylength months of October through April was greatest for guineagrass, intermediate for bermuda-
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Table 1 Light and species means for total annual DM harvested at South Florida, and light × specie interaction for Puerto Rico and St. Croix Species
South Florida
Puerto Rico
St. Croix
Natural (Mg ha− )
Extended (Mg ha− )
P-value
Natural (Mg ha− )
Extended (Mg ha− )
P-value
Natural (Mg ha− )
Extended (Mg ha− )
P-value
Bahiagrass Guineagrass Bermudagrass
0.14 1.31 0.59
0.49 1.50 0.93
<0.01 <0.01 <0.01
0.85 4.10 2.23
1.64 4.22 2.15
<0.01 0.52 0.62
0.56 3.19 1.26
0.80 3.23 1.19
0.18 0.80 0.66
Mean
0.68b
0.98a‡
0.04
Data are means averaged across 14 harvests (October–April in 2003 and 2004) and 6 replicates (n = 84). ‡ Daylength means within a row not followed by the same letter are different (P < 0.01).
grass, and lowest for bahiagrass (1.4, 0.76, and 0.32 Mg ha−1 , respectively). Daylength effects (Table 1) were present at this location for all three grasses (P < 0.01). Averaged across grasses, exposure to extended daylength resulted in an average 44% increase in DM harvested during the short-daylength months from 0.68 Mg ha−1 for natural daylength to 0.98 Mg ha−1 for the extended photoperiod. Specifically, DM harvested for bahiagrass increased (P < 0.01; Table 1) from 0.14 Mg ha−1 under natural daylength to 0.49 Mg ha−1 under extended daylength, which amounted to an increment of 3.5 times. Harvested DM for guineagrass increased 15%
from 1.31 Mg ha−1 for the natural daylength treatment to 1.50 Mg ha−1 for the extended daylength treatment. For bermudagrass, the increase was 58% from 0.59 Mg ha−1 under natural daylength compared to 0.93 Mg ha−1 when grown under extended photoperiod. At Puerto Rico, only bahiagrass showed a significant increase in DM when exposed to extended daylength (P < 0.01; Table 1). Bahiagrass DM harvested during October through April under extended daylength was 92% greater than for natural daylength (1.64 Mg ha−1 versus 0.85 Mg ha−1 ). Daylength had no influence on guineagrass (P = 0.52) or bermudagrass (P = 0.62)
Fig. 2. Bahiagrass dry matter (DM) harvested means under extended- and natural-daylength treatments for the locations of south Florida, Puerto Rico and St. Croix. The symbols represent the significance of the treatments at each harvest for the winter season 2002–2003 and 2003–2004 (NS, not significant, † P < 0.1; * P < 0.05; ** P < 0.01).
Fig. 3. Guineagrass dry matter (DM) harvested means under extended- and natural-daylength treatments for the locations of south Florida, Puerto Rico and St. Croix. The symbols represent the significance of the treatments at each harvest for the winter season 2002–2003 and 2003–2004 (NS, not significant, † P < 0.1; * P < 0.05; ** P < 0.01).
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DM harvested. Harvested DM of guineagrass was 4.10 and 4.22 Mg ha−1 and 2.23 and 2.15 Mg ha−1 for bermudagrass for natural and extended daylength, respectively. In St. Croix, when simple effects were explored, species × daylength effects approached significance (Table 1). Bahiagrass DM increased 43% from 0.56 Mg ha−1 when grown under natural daylength to 0.80 Mg ha−1 under extended (P = 0.18); but similar to Puerto Rico, guineagrass and bermudagrass did not show any response to daylength at this location (3.19 Mg ha−1 versus 3.23 Mg ha−1 and 1.26 Mg ha−1 versus 1.19 Mg ha−1 , respectively, for guineagrass and bahiagrass under natural and extended daylength).
February and March 2004. Harvest data in St. Croix presented greater experimental errors compared to the other locations that did not allow detection of statistical differences. Those errors were associated with episodes of intermittent operational problems with the forced-air oven dryer. Increased DM of guineagrass with extended daylength (Fig. 3) occurred only in south Florida for the November and December 2002 harvests and in St. Croix for the January 2003 harvest. Bermudagrass had increased DM with extended daylength (Fig. 4) only in south Florida from November 2002 to March 2003 but the effect was not consistent in the harvests of October 2003–April 2004.
3.2.2. Light effects and seasonal distribution of DM harvested Daylength effects on DM harvested for bahiagrass (Fig. 2) were reflected at each harvest from November 2002 to April 2003, and December 2003 to April 2004. Daylength effects on DM harvested in Puerto Rico were also observed at all harvests from October 2002 to April 2003, and November 2003 to April 2004. Dry matter harvested at St. Croix showed daylength effects from October 2002 to March 2003, but the effects were not present during most of the short-daylength months of the 2nd year (October 2003 to January 2004) and were observed only in
3.3. Herbage nutritive value
Fig. 4. Bermudagrass dry matter (DM) harvested means under extended- and natural-daylength treatments for the locations of south Florida, Puerto Rico and St. Croix. The symbols represent the significance of the treatments at each harvest for the winter season 2002–2003 and 2003–2004 (NS, not significant, † P < 0.1; * P < 0.05; ** P < 0.01).
3.3.1. Daylength effects for CP and IVDOM There were strong location effects (P = 0.02) and interactions with location (P < 0.01) for CP and IVDOM, respectively; there-
Fig. 5. Crude protein (CP) concentration for bahiagrass, guineagrass and bermudagrass herbage harvested at South Florida (A), Puerto Rico (B), and St. Croix (C). Data are means averaged across two harvests (11 November 2002, and 11 November 2003) and two replicates (n = 4). The symbols represent the significance of the treatments at each harvest for the winter season 2002–2003 and 2003–2004 (NS, not significant, † P < 0.1; * P < 0.05).
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In contrast to bahiagrass, there was no effect of daylength on herbage CP of guineagrass or bermudagrass. This lack of effect was similar across all locations. The averages for natural versus extended daylength were 119 g kg−1 versus 131 g kg−1 for guineagrass (P = 0.24) and 136 g kg−1 versus 130 g kg−1 for bermudagrass (P = 0.59), respectively. Comparison of IVDOM when analyzed by location, showed a response to daylength at St. Croix (Fig. 6C). There was a significant increase in IVDOM of guineagrass in response to the extended daylength as compared to the natural daylength (608 g kg−1 versus 481 g kg−1 , respectively). In contrast, the IVDOM of bahiagrass grown under extended daylength in St. Croix was lower relative to the natural daylength treatment (463 g kg−1 versus 538 g kg−1 , respectively, P = 0.11). At south Florida and Puerto Rico there was no daylength effect on IVDOM for any species. 4. Discussion and conclusions
Fig. 6. In vitro digestible organic matter (IVDOM) for bahiagrass, guineagrass, and bermudagrass herbage harvested at South Florida (A), Puerto Rico (B), and St. Croix (C). Data are means averaged across two harvests (11 November 2002, and 11 November 2003) and two replicates (n = 4). The symbols represent the significance of the treatments at each harvest for the winter season 2002–2003 and 2003–2004 (NS, not significant, † P < 0.1).
fore, the data are presented by location (Figs. 5 and 6). Herbage CP concentration had a daylength × species interaction effect (P < 0.01). Interaction occurred because herbage CP concentrations of bahiagrass were significantly (P < 0.01) higher for plants exposed to natural daylength than for plants exposed to extended daylength. The herbage CP concentrations of bahiagrass for natural versus extended daylength treatments were 136 g kg−1 versus 115 g kg−1 at south Florida, 140 g kg−1 versus 104 g kg−1 at Puerto Rico, and 157 g kg−1 versus 109 g kg−1 at St. Croix. Across locations, the difference between daylength treatments amounted to an average decrease of 35 g CP kg−1 for the extended daylength treatment (109 g kg−1 ) as compared to the natural daylength treatment (144 g kg−1 ). Lower CP values of bahiagrass under extended daylength may correspond to a dilution effect. Given the same CP content in the tissue, the DM harvested was lower for the natural daylength but was greater under extended photoperiod.
During short days, forage DM harvested of bahiagrass, guineagrass, and bermudagrass varied depending on the experimental location. Bahiagrass consistently had greater DM harvested across all three locations in response to extended daylength during October through April. The positive response in St. Croix with a lower fertilizer application proved that if photoperiod had an influence even minimum fertilizer should allow a positive response as was the case in this location. The findings from these three locations corroborate the considerable sensitivity of bahiagrass to photoperiod that has been reported in subtropical areas (Marousky et al., 1991; Newman et al., 2004; Sinclair et al., 2001, 2003). In contrast to bahiagrass, guineagrass and bermudagrass had significant increases in DM harvested under extended daylength only at the subtropical site (south Florida). The increase in DM with extended daylength for guineagrass and bermudagrass that was present in south Florida was not observed at lower, tropical latitudes (Puerto Rico and St. Croix; P > 0.52). The failure to alleviate in guineagrass and bermudagrass at least part of the decline in growth during the short-daylength period with extended daylength implies that there are other factors involved in growth regulation of these two grasses. In terms of nutritive value, CP of bahiagrass was consistently lower at all locations under the extended daylength treatment. Lower CP of bahiagrass growing under extended daylength compared to those growing under natural daylength plots is at least partially accounted for by a dilution effect due to the greater DM harvested under extended light. These results corroborate previous results with bahiagrass at higher latitude (Sinclair et al., 2003). The CP in either case was not marginal and was above the minimum critical levels (60–80 g kg−1 ) required for livestock production and proper rumen-bacteria functioning (Humphreys, 1991; Van Soest, 1994). In vitro digestible organic matter was only affected by extended daylength at St. Croix. The onset of plant reproduction has been recognized as a crucial factor in C4 grass herbage quality decline (Coleman et al., 2004). Given the potential of plant development stage to affect nutritive value and yield, a detailed analysis of the plant development pattern in
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response to daylength seems warranted to fully understand the observed responses. The implications of our results are that bahiagrass is capable of producing additional forage in the winter season if sensitivity to photoperiod is removed or attenuated, but this does occur at the expense of herbage CP concentration. Thus, there is room for increasing the production of bahiagrass and the challenge lies in selecting and developing photoperiod insensitive plants of this species. Acknowledgments The authors thank are grateful for the participation and field assistance of Mr. Hector Torres and Mr. Stuart Weiss (research associates in Puerto Rico and St. Croix Research Stations, respectively), and the field crew at Immokalee Research Station for their cooperation and sustained effort in maintaining and harvesting the plots which were key aspects in this long-term experiment. References AOAC, 1990. Official Methods of Analysis, 15th ed. AOAC, Arlington, VA. Burton, G.W., 1967. A search for the origin of Pensacola bahia grass. Econ. Bot. 21, 379–382. Coleman, S.W., Moore, J.E., Wilson, J.R., 2004. Quality and utilization. In: Moser, L.E., et, al. (Eds.), Warm-season C4 Grasses. ASA/CSSA/SSA, Madison, WI, pp. 267–308. Gallaher, R.N., Weldon, C.O., Futral, J.G., 1975. An aluminum block digester for plant and soil analysis. Soil Sci. Soc. Am. Proc. 39, 803–806. Henry, J.A., Portier, K.M., Coyne, J., 1994. The Climate and Weather of Florida. Pineapple Press Inc., Sarasota, FL.
Humphreys, L.R., 1991. Tropical Pasture Utilization. Cambridge University Press, Cambridge. Littell, R.C., Freund, R.J., Spector, P.C., 2002. SAS System for Linear Models. SAS Inst., Cary, NC. Marousky, F.J., Ploetz, R.C., Clayton, D.C., Chambliss, C.G., 1991. Flowering response of Pensacola and Tifton 9 bahiagrasses grown at different latitudes. Soil Crop Sci. Soc. Fla. Proc. 50, 65–69. Moore, J.E., Mott, G.O., 1974. Recovery of residual organic matter from in vitro digestion of forages. J. Dairy Sci. 57, 1258–1259. Newman, Y.C., Sollenberger, L.E., Boote, K.J., Allen Jr., L.H., Littell, R.C., 2001. Carbon dioxide and temperature effects on forage dry matter production. Crop Sci. 41, 399–406. Newman, Y.C., Sinclair, T.R., Blount, A.R., Lugo, M.L., Valencia, E., 2004. Flowering of tropical forages exposed to extended photoperiod. In: 2004 Agronomy abstracts. ASA, Madison, WI. Ramos-Santana, R., Rodriguez-Arroyo, J.E., 1991. Seasonal production of 11 Panicum maximum cultivars harvested at a 45-day interval. J. Agric. Univ. P. R. 75, 61–66. SAS Institute Inc., 2004. SAS/STAT 9.1 User’s guide. SAS Institute Inc., Cary, NC. SAS Institute Inc., 1996. SAS/STAT Software: Changes and Enhancements Through Release 6.11. SAS Institute Inc., Cary, NC. Sinclair, T.R., Mislevy, P., Ray, J.D., 2001. Short photoperiod inhibits winter growth of subtropical grasses. Plantarum 213, 488–491. Sinclair, T.R., Ray, J.D., Mislevy, P., Premazzi, L.M., 2003. Growth of subtropical forage grasses under extended photoperiod during short-daylength months. Crop Sci. 43, 618–623. Sinclair, T.R., Ray, J.D., Premazzi, L.M., Mislevy, P., 2004. Photosynthetic photon flux density influences grass responses to extended photoperiod. Environ. Exp. Bot. 51, 69–74. Sinclair, T.R., Newman, Y.C., Lugo, M.L., Valencia, E., Blount, A.R., 2005. Potential for year-round forage production in Puerto Rico and St. Croix. J. Agric. Univ. P. R. 89, 133–148. Van Soest, P.J., 1994. Nitrogen metabolism. In: Van Soest, J.P. (Ed.), Nutritional Ecology of the Ruminant. Cornell University Press, Ithaca, New York.