Ruminant Intake, Digestibility, and Nitrogen Balance of Gamagrass Hay and Baleage Harvested During Sunrise and Sunset
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2006-05-08
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Abstract
Two experiments were conducted to evaluate the differences in TNC composition of gamagrass harvested in the afternoon (PM) compared to the morning (AM). In the first experiment gamagrass hay was fed to goats with half receiving a protein supplement while the other half received none in a crossover design with a 2 X 2 factorial arrangement of treatments. In the second experiment gamagrass baleage fertilized at 2 different nitrogenous levels 50 kg N/ha (LO) and 150 kg N/ha (HI) was fed to steers in a similar experimental design with half of the steers receiving LO and the other HI. In both experiments TNC concentration including monosaccharides was increased in PM compared to AM (P < 0.001). During the ad libitum phase, goats and steers both had similar DMI of PM and AM (P < 0.88). There was a tendency (P < 0.14) for digestible DMI (DDMI) to be increased by 4 % for PM compared to AM in the goat study; however, was decreased by 9 % in the steer study. During the balance trial, there was a tendency (P < 0.09) for goats to have increased hay and total (hay + supplement) intakes of PM compared to AM, but there were no differences for steers. DDMI was 7 % greater for PM than AM in the goat study, whereas the opposite held true for the steers. DDMI was 9.5 % greater for AM than PM. Compared to AM, PM improved dry matter digestibility (DMD) (55.7 vs. 53.1% DM) when fed to goats. There was a tendency for DMD to be improved in AM (53.2 vs. 51.0 % DM) when fed to steers. In both experiments, the higher protein level (supplement and HI) improved DMD, although the supplement did so to a greater extent. Goats and steers fed supplement and HI had increased N intakes and N digestibility (P < 0.02). In both experiments, the higher protein levels increased urinary N, urine urea N, and urea N as a percent of total urinary N (P < 0.001). Compared to no supplement and LO, supplementation and HI increased plasma urea N (P < 0.05), with overall levels being higher in goats (13.24 vs. 8.61 mM) than steers (6.04 to 4.12 mM). Due to all of these factors N retention was improved with supplementation (2.3 vs. 1.2 g/d), but was similar (P < 0.89)
between fertilization levels. We concluded from these results that PM forages do have increased TNC concentration and the differences are preserved through the drying process of making hay and fermentation during ensiling. However, ruminants do not always prefer the PM forages, especially when they are ensiled. N retention in goats can be improved with supplementation of low CP forages. To investigate the mechanism that resulted in an increased preference or DMI of PM forages we measured insulin and ghrelin concentrations. There was a tendency for insulin to be elevated in AM compared to PM (P < 0.12) and HI compared to LO (P < 0.07). Across all treatments, ghrelin concentration peaked at 1000 and decreased for the remainder of the day. We concluded from this data that ghrelin concentration in beef steers increases prior to feeding (animals were fed at 1000), which supports other data suggesting that it may be involved in meal anticipation. My hypothesis was that ghrelin and insulin concentrations would be affected by differences in macronutrient intake, specifically TNC. In the steer study, AM increased DMI, DDMI, and DMD, and the differences in plasma hormone concentration may suggest that regulation of intake between time of harvest may differ as well.
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Total non-structural carbohydrates, nitrogen metabolism, goats, steers, tripsacum dactyloides, voluntary intake
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Degree
MS
Discipline
Nutrition
