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Estimation of feeding value of four tropical forage species at two stages of maturity^1,2

University of Florida-IFAS, Range Cattle Research and Education Center, Ona 33865

	Abstract

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The feeding value of four tropical grasses was assessed through voluntary intake and digestibility studies using yearling Brahman x British steers (average BW = 256 ± 34 kg). The digestibility of OM was estimated using total fecal collection (TFC), in vitro OM digestibility (IVOMD), and by estimating fecal production using insoluble acid detergent fiber (IADF) as an indigestible marker. The four grasses consisted of bahiagrass (Paspalum notatum), limpograss (Hemarthria altissima), bermudagrass (Cynodon dactylon), and stargrass (Cynodon spp.). Grass was harvested at two stages of maturity (approximately 4 and 10 wk). Forages were ground (5 to 10 cm) and offered to steers ad libitum. Forage treatments were assigned randomly to steers over eight 28-d periods and repeated over two consecutive years. Total forage offered and refused was determined during a 14-d sample collection period. For determination of fecal output, steers were placed into metabolism crates for 7 d. Composited samples of forage offered, forage refused, and feces of each steer at each period were analyzed for DM, OM, NDF, ADF, IADF, IVOMD, and CP. All digestibility results were calculated on an OM basis. There were year x grass x maturity interactions (P < 0.01) for all measures of forage quality, except CP. Increased maturity resulted in a 37.8% decrease (P < 0.001) in CP concentration when averaged across all forages. Four-week bermudagrass contained the greatest (P < 0.05) concentration of CP compared with all other grasses at both maturities, except 4-wk stargrass. Bahiagrass IVOMD did not differ among 4- and 10-wk maturities in both years; however, the IVOMD content of both stargrass and bermudagrass decreased (P < 0.05) when these forages matured from 4 to 10 wk. Apparent OM digestibility, determined by TFC, was greater (P < 0.05) than OM digestibility determined by IVOMD and IADF for all forages except bahiagrass, for which IADF did not differ from TFC. In Year 1, OM intake (OMI) of 10-wk limpograss was less (P < 0.05) than all other 4-wk forages. In Year 2, voluntary OMI of 10-wk limpograss was less (P < 0.05) than all grass x maturity combinations, except for 10-wk bermudagrass. These data suggest that important differences exist in changes in nutrient quality associated with increased maturity in tropical forages. Among the forages assessed in this study, bahiagrass seems to better retain nutrient quality when maturing from 4 to 10 wk.

Key Words: Bahiagrass • Bermudagrass • Insoluble Acid Detergent Fiber • Limpograss • Stargrass

	Introduction

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Two of the most influential factors affecting forage quality and utilization are forage species and forage maturity. Compared with temperate forages, tropical forages typically have increased annual DM yield; however, increased yield is usually associated with decreased forage quality and subsequent feeding value to cattle (Skerman and Riveros, 1990). Forage quality has a major effect on the productivity and utilization of the forage-producing land base (Allen and Segarra, 2001). Cattle supplementation needs within forage-based systems are a direct response of the presence or lack of adequate forage nutrients (Kunkle et al., 1999). In Florida, the primary pasture forage is bahiagrass (Paspalum notatum), covering approximately 1 million ha (Chambliss and Sollenberger, 1991). Other tropical forages are also utilized in Florida, with three of the most common forages being limpograss (Hemarthria altissima), bermudagrass, (Cynodon dactylon), and star-grass (Cynodon spp.). A summary of the characteristics of these forages, as well as bahiagrass, is available (Chambliss, 1999). To our knowledge, the chemical composition and feeding value of these forages have never been compared in a single study where growing conditions and fertilization practices were standardized across all forage types. The current study was designed to test our hypothesis, which stated that significant differences exist in the feeding value, relative to stage of maturity, of these commonly used tropical forages. A secondary objective of this research was to examine the accuracy of using insoluble acid detergent fiber (IADF) as an internal marker for estimating forage intake. The results of this research will assist cattle producers within the subtropics to better utilize various pasture forage options. In addition, these data will assist in the development of supplementation programs designed to fortify the nutritional needs of cattle consuming these forages.

	Materials and Methods

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Animals, Treatments, and Sample Collection

Steers used in these experiments were cared for by acceptable practices (FASS, 1999) for the care and use of agricultural animals in agricultural research, and the protocol was approved by the University of Florida Institutional Animal Care and Use Committee. This study was conducted over two consecutive years (1999 and 2000) at the University of Florida, Range Cattle Research and Education Center, Ona (long 27°23.88 N; lat 081°56.20 W). Four forages, ‘Pensacola’ bahiagrass, ‘Floralta’ limpograss, ‘Tifton 85’ bermudagrass, and ‘Florona’ stargrass were harvested from established forage stands (4 yr). Forages were harvested during the preceding fall for each year of the study. Forages were initially cut to ground level (approximately 4 cm) in early October, residue was removed, and then the forages were allowed to regrow concurrently for approximately 4 and 10 wk, at which time they were cut again at ground level and cured and harvested into round bales (approximately 375 kg). Precipitation during these growing periods was approximately 7.4 and 18.9 cm, and 6.0 and 14.9 cm for the 4- and 10-wk maturities in Years 1 and 2, respectively. To remove the potential interaction of N application x forage species on measures of forage quality (Johnson et al., 2001), no fertilizer was applied to pastures used to obtain experimental hays. All hays were harvested within 7 d of cutting without being rained on. Before the start of the study, dried forage was stored under cover. Before feeding, the bales were ground to pass a 2.5-cm screen using a commercial tub-style hay grinder and stored under cover. To determine the effect of forage species and maturity on voluntary intake, each forage x maturity treatment was evaluated in six growing steers each year (approximately 16 mo of age; Brahman x British crossbred; average BW = 256 ± 34 kg). Steers were housed individually in covered pens (15 m²). Forage treatments were offered ad libitum by providing a daily amount of forage exceeding the previous day’s intake by at least 15%. Before the start of the experiment each year, six of the eight treatments were assigned randomly to each of eight 28-d periods, such that each steer was assigned each forage treatment at least once. In each period, total forage offered and refused was determined during a 14-d sampling period for voluntary forage intake, which immediately followed a 7-d diet adaptation period. Individual animal BW was determined at the start and end of each period. Free-choice access to trace mineral-containing salt blocks was available to all steers throughout the study.

To estimate apparent forage OM digestibility, total fecal output was measured in all steers during a 7-d fecal collection period, which immediately followed the forage intake determination period. Steers were placed into metabolism stalls (1 m x 3 m) and total feces produced were measured. During this 7-d collection period, daily samples of forage offered, forage refused, and feces were collected and composited. Subsamples were dried in a forced-air oven at 50°C, ground to pass a 1-mm screen, and analyzed for DM and OM (AOAC, 1990). Subsamples also were retained for analysis of CP, NDF, ADF, ADL, IADF, and in vitro OM digestibility (IVOMD).

Determination of Forage Quality

Duplicate forage, orts, and fecal samples were analyzed for DM, ash, and total N (Kjeldahl method) according to AOAC (1990) procedures. Duplicate samples for NDF and ADF and triplicate samples for ADL were analyzed using methods described by Goering and Van Soest (1970). In vitro OM digestibility was determined by the Moore and Mott (1974) modification of the Tilley and Terry (1963) procedure. Ruminal fluid was collected from one mature, ruminally fistulated steer (approximately 500 kg; Brahman x British crossbred) fed star-grass hay ad libitum, plus 0.5 kg (as-fed basis) of soybean meal (Glycine max (L.) Merr.) daily. All IVOMD analyses were conducted in duplicate runs with three samples per run.

Analysis of IADF was conducted using modified procedures from Goering and Van Soest (1970). Briefly, a 0.6-g sample was placed into a 50-mL glass centrifuge tube with 30 mL of ruminal fluid inoculum and buffer (50:50; McDougall, 1948). A two-stage IVOMD procedure was performed as described by Moore and Mott (1974). Following pepsin removal, an additional in vitro digestion was performed on the remaining particulate matter using 30 mL of the 50:50 mixture of ruminal fluid inoculum and buffer for 96 h. Following digestion, samples were centrifuged (1,500 x g for 15 min), and the supernatant fraction was removed. Contents were transferred to a 600-mL Berzelius beaker and rinsed with ADF solution, bringing the volume to 150 mL. An ADF extraction was performed, and IADF was calculated as follows: ([ADF of residue OM – blank]/[sample OM weight]) x 100. An estimate of fecal output was calculated by determining the percentage of IADF disappearance as follows: IADF of OMI/IADF of feces OM.

Statistical Analyses

Analysis of variance was performed using PROC GLM of SAS (SAS Inst., Inc., Cary, NC) for an incomplete Latin square design evaluating eight forage treatments (four forages x two maturities) using six individual steers within eight 28-d periods. The study was replicated over two consecutive years. Allocation of forage treatments was predetermined before the start of the study, such that all steers received each forage treatment at least once. The model statement included effects of year, forage species, forage maturity, and all possible interactions. The model statement also included steers nested in years, period, and the period x year interaction. The F-tests were conducted from a random statement containing steer nested in years with a test option. When F-tests were significant (P 0.05), differences among individual least squares means were separated.

	Results and Discussion

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Forage Crude Protein

Averaged over all grasses, increased forage maturity (10-wk regrowth) was associated with 37.8% less CP concentration (P < 0.001) compared with harvesting at 4-wk regrowth. Significant forage x maturity interactions were detected (P < 0.001; Figure 1), which seemed to result from differences in order of magnitude and not from differences in treatment ranking. Other researchers have reported that average tropical forage CP content decreases below 9% after 6 wk of summer regrowth (Brown and Mislevy, 1991). In the current study, 4-wk bermudagrass had a greater (P < 0.05) CP content than bahiagrass and limpograss. Mislevy and Martin (1998) previously compared the CP content of the two Cynodon forage species used in the current study (stargrass and bermudagrass) over three consecutive years. In their study, CP content was less in star-grass vs. bermudagrass harvested in April following winter and spring accumulation; however, this difference was not observed when bermudagrass was allowed to mature an additional month.

View larger version (22K): [in this window] [in a new window]   Figure 1. Effect of grass species and maturity (4- and 10-wk regrowth) on CP concentration. a,b,c,d,eMeans with different superscripts differ, P < 0.05. Pooled SEM = 0.42. Table values are least squares means (n = 12 observations per forage species x maturity).

Forage Fiber (NDF, ADF) and Lignin

There was a grass x maturity x year interaction for NDF, ADF, and ADL concentrations (P < 0.05; Table 1). In both years, NDF concentration was least (P < 0.05) in bahiagrass compared with all other grasses, except 4-wk stargrass in Year 2. In Year 2, bahiagrass was the only grass that did not increase in NDF concentration when maturing from 4 to 10 wk.

Forage Intake

In Year 1, 10-wk limpograss provided a lesser (P < 0.05) voluntary OMI compared with all other 4-wk forages. In Year 2, voluntary OMI of 10-wk limpograss was less (P < 0.05) than all grass x maturity combinations, except 10-wk bermudagrass (Table 2). These results are similar to those reported by Moore et al. (1981). In that study, the quality of 12 tropical grass hays was evaluated at three or four stages of maturity. Using sheep, their results showed a tendency for a decrease in forage OM intake as maturity increased. As well, voluntary OM intake was less (P < 0.02) for limpograss compared with all other forages (1.42 vs. 1.82% BW; SEM = 0.08).

In both Years 1 and 2, stargrass and bermudagrass, but not bahiagrass, experienced a decrease (P < 0.05) in IVOMD when maturing from 4- to 10-wk (average IVOMD decrease = 1.1, 9.9, and 6.2% for bahiagrass, bermudagrass, and stargrass, respectively; Table 1). This decrease in IVOMD for the Cynodon grasses is less than that observed by Mislevy and Martin (1998), who reported a 10.8% average decrease in IVOMD when forages were grazed at 7- vs. 2-wk intervals. In agreement with the decrease in ADF and lignin content, limpograss IVOMD concentration increased (P < 0.001) from 4- to 10-wk maturity in Year 2, supporting the proposed presence of residue from the initial cutting and clearing, as discussed previously. Of the Cynodon grasses, 4-wk bermudagrass maintained a greater (P < 0.001) IVOMD during both years than stargrass (Table 1). These results are similar to those in a study reported by Mislevy and Martin (1998), where winter and spring accumulation of bermudagrass had a greater (P < 0.05) IVOMD than stargrass over two consecutive years. Johnson et al. (2001) reported a greater IVOMD for bermudagrass compared with both bahiagrass and stargrass (nonfertilized, 4-wk regrowth).

Apparent forage OM digestibility of the Cynodon grasses measured by TFC tended to be decreased (P < 0.07) with increasing maturity in both years (Table 2). In comparison, bahiagrass experienced no decrease (P > 0.29) in apparent digestibility when comparing 4- and 10-wk maturities (2.5% average decrease in apparent digestibility; via TFC; Table 2). These results are similar to those reported by Brown and Mislevy (1988; same research location as the current study), in which star-grass IVOMD concentrations were less than bahiagrass when both forages were cut after 6-wk of summer re-growth. The decrease in digestibility observed in the Cynodon grasses is likely a response of increased forage yield during the summer months compared with bahiagrass (Brown and Mislevy, 1988). Similar to IVOMD, apparent digestibility of limpograss (via TFC) increased (P < 0.001) from 4- to 10-wk maturity in Year 2 (Table 2). These data imply that bahiagrass suffers minimal decreases in both in vitro and in vivo OM digestibility when maturing from 4- to 10-wk. In comparison, the tropical Cynodon species (bermudagrass and stargrass) suffer greater losses in feeding value as they mature.

There was an interaction (P < 0.05) between grass type and method for estimating forage OM digestibility in growing steers. Apparent OM digestibility determined by TFC was greater (P < 0.05) than OM digestibility determined by IVOMD and IADF for all forages except bahiagrass, for which IADF did not differ from TFC (Table 3). Averaged over all forages, IVOMD and IADF underestimated (P < 0.03) forage OM digestibility by 11.6% compared with TFC. Sunvold and Cochran (1991) also reported differences among methods for determination of forage OM digestibility. Similar to the results of the current study, their results also showed that IADF underestimated forage OM digestibility as determined by total fecal collection. Their study used temperate forages (alfalfa, bromegrass, and prairie grass hays), suggesting that IADF, as an internal marker, may underestimate OM digestibility similarly in tropical and temperate forages.

	Footnotes

 
¹ Contribution No. R-10571 from the Florida Agric. Exp. Stn. This research was supported in part by a grant from the USDA Tropical–Subtropical Agric. Res. Program.

² Appreciation is expressed to C. Piacitelli and T. Wood for technical assistance on these experiments.

⁴ Current address: Florida Agric. Exp. Stn., 1022 McCarty Hall, Gainesville, FL 32611-8100.

³ Correspondence: 3401 Experiment Stn. (phone: 863-735-1314; fax: 863-735-1930; e-mail: jdarthington{at}mail.ifas.ufl.edu).

Received for publication September 22, 2004. Accepted for publication March 17, 2005.

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