Effects of total nonstructural carbohydrates and nitrogen balance on voluntary intake of goats and digestibility of gamagrass hay harvested at sunrise and sunset
Introduction
Harvest of tall fescue (Festuca arundinacea Schreb.), alfalfa (Medicago sativa L.), and switchgrass (Panicum virgatum L.) in the PM versus AM increased total nonstructural carbohydrate (TNC) concentration in hays (Fisher et al., 1999, Fisher et al., 2002, Fisher et al., 2005, Huntington and Burns, 2008). Forages with increased TNC had higher apparent dry matter (DM) digestibility when offered to goats (Burns et al., 2005) or steers (Huntington and Burns, 2008). Cattle, sheep and goats preferred PM harvested tall fescue (Fisher et al., 1999) and alfalfa (Fisher et al., 2002) when pairs of AM harvested and PM harvested hays were offered side-by-side in preference studies. In these short-term studies, preference was measured as an increase in DM intake over several meals. Based on these results, and others (i.e., Buntinx et al., 1997, Orr et al., 1997, Mayland et al., 2000), the increased preference for forages harvested or grazed in the PM is attributable to the increase in the TNC fraction. Steers fed PM harvested versus AM harvested alfalfa hay (Burns et al., 2005), or gamagrass (Tripsacum dactyloides (L.) L.) preserved as baleage (forage ensiled in large, round bales covered with plastic, Huntington and Burns, 2007), had increased DM intake over a 3-week period. However, time of harvest did not affect DM intake of steers fed switchgrass hay harvested in the PM or AM (Huntington and Burns, 2008).
Increased dietary protein supply, specifically increased supply of ruminally degradable protein, increased apparent digestibility of crude protein (CP) with warm-season grass hay or pasture (Olson et al., 1999, Bodine et al., 2000, Bodine and Purvis, 2003, Huntington and Burns, 2008). Further, synchronization of degradable protein and fermentable carbohydrates increased ruminal microbial yield in vitro (Argyle and Baldwin, 1989), production of organic acids in vitro (Hall and Weimer, 2007) and in vivo ruminal concentrations of organic acids in dairy cattle (Aldrich et al., 1993).
The first objective was to determine effects of time of harvest on TNC concentrations in gamagrass hay and to determine if PM harvest increased intake and digestibility of gamagrass hay when it was fed to goats. The second objective was to assess the interaction between protein supplementation and time of harvest on intake and N balance of goats.
Section snippets
Animal and experimental procedures
The experimental design was a crossover in a 2 × 2 factorial arrangement of treatments. Two experimental treatments were derived from one field (0.81 ha) of Iuka gamagrass (Tripsacum dactyloides (L.) L.), which was divided into four strips with time of harvest assigned randomly to each strip. The gamagrass was regrowth from an earlier cutting, and was in late vegetative stage at harvest. On 24 August 2003, the PM strips were cut with a mower conditioner at 18:00 h. The day was characterized as dry (
Results
Except for the following few variables, there were no interactions between time of harvest and supplementation, so only main effects of harvest and supplementation are presented in tables. There was a slightly higher (P<0.03) response in N digested when supplement was added to the AM harvest than when supplement was added to the PM harvest, which was associated with a similar trend (P<0.08) in N intake. Intake of monosaccharides during the ad libitum intake phase decreased (P<0.05) when
Discussion
Although there were differences in NDF, ADF and cellulose composition between the AM and PM harvests (Table 2), they were not likely biologically relevant because differences between harvests in these fiber components were 10 g/kg of DM or less and intakes of NDF, ADF, cellulose and lignin (sa) did not differ between the AM and PM harvests (Table 4, Table 5). Others have reported that PM harvest decreases (Fisher et al., 1999), or has no effect (Orr et al., 2001, Fisher et al., 2005, Burns et
Conclusions
Compared to an AM harvest, PM harvest of gamagrass hay had 5% higher DM digested that resulted in a 6% increase in digestible DM intake. Approximately 0.33 of increased digestible DM intake from the PM harvest was attributable to an increased TNC concentration in the PM hay. Protein supplementation had small effects on intake and apparent digestibility of N, NDF and ADF of gamagrass.
Acknowledgments
Authors thank Ms. Lucile Ganey and Ms. Ellen Leonard for their able laboratory work and Ms. Sharon Freeman and her staff at the Metabolism Education Unit for care and feeding of the goats.
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