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Title: The Effect of Total-Nonstructural Carbohydrates and Nitrogen Balance on Voluntary Intake of Goats and Digestibility on Gamagrass Hay Harvested at Sunrise and Sunset

item SAUVE, A
item Burns, Joseph

Submitted to: Animal Feed Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2008
Publication Date: 7/17/2008
Citation: Sauve, A.K., Huntington, G.B., Burns, J.C. 2008. The Effect of Total-Nonstructural Carbohydrates and Nitrogen Balance on Voluntary Intake of Goats and Digestibility on Gamagrass Hay Harvested at Sunrise and Sunset. Animal Feed Science And Technology. 148:93-106.

Interpretive Summary: Previous research has shown that forage cut in the late afternoon (PM) and preserved as hay was generally preferred by ruminants over hay that was cut in the early morning (AM). Intake trials with steers has shown with cool-season grasses and with alfalfa that ruminants frequently consumed more or digested more of the dry matter from PM cut hay. This study examined the effect of time-of-cut on the total nonstructural carbohydrate (TNC) concentrations in the warm-season grass, gamagrass, and to determine if N concentration in the rumen, via N supplementation, would enhance carbohydrate capture. Cutting gamagrass in the PM resulted in greater concentrations of TNC in the forage and when fed to goats greater dry matter digestion and digestion of the cell wall (NDF) as well as its constituent fiber fractions of acid detergent fiber, and cellulose. Although dry matter intake was not altered, digestible dry matter intake was greater for the PM hay. The addition of N also increased dry matter digestion and the digestion of the cell wall, but the effect was small. Adopting the practice of cutting a warm-season grass for hay in the late PM following a sunny day may have economic value as essential no additional cost is incurred, but improved utilization efficiency by the animal can be expected.

Technical Abstract: We evaluated the differences in composition of Iuka gamagrass (Tripsacum dactyloides L.) hay harvested at 0530 (AM harvest) or 1730 (PM harvest), and measured how protein supplementation and time of harvest interact to affect the voluntary intake, digestibility, and N balance of goats. Boer X Spanish wethers (N = 28; 24 ± 3 kg) were randomly assigned to receive supplement (31% CP, fed at 11% of dry matter (DM) intake, 14 goats) or no supplement (14 goats). Within supplement or no supplement groups, goats were assigned randomly to a crossover design of AM harvest (7 goats) or PM harvest (7 goats). Goats were housed individually in metabolism crates with free access to water and mineral blocks. They were fed twice daily, with supplement offered 30 min prior to the morning feedings. After a 7-d adaptation, voluntary intake (goats were offered 110% of previous day’s intake) was measured for 14 d, followed by a 4-d adjustment phase (to equalize DM offered between periods) and a 5-d digestion and balance phase. After Period 1, the goats were switched to their new hay harvest times, and the protocol was repeated. Compared to AM harvest, the PM harvest had greater (P< 0.03) proportions of total nonstructural carbohydrates (TNC, 70.8 vs. 59.0 g/kg DM), monosaccharides (37.0 vs. 28.6 g/kg DM), di- and polysaccharides (18.5 vs. 15.4 g/kg DM) and less NDF (700 vs 710 g/kg). Crude protein (79 g/kg DM) and starch (15.2 g/kg DM) were similar (P<0.88) for PM harvest and AM harvest. Dry matter digestibility was greater (P<0.03) for PM harvest vs. AM harvest (555 vs. 531 g/kg DM) and for supplement vs. no supplement (563 vs. 522 g/kg DM). Voluntary gamagrass DM intake (550 vs. 548 g/d, P<0.90) and calculated total digestible DM intake (327 vs. 313 g/d, P<0.14) were similar for PM harvest and AM harvest; however, total digestible DM intake during the digestion and balance phase was greater (P<0.01) for PM harvest vs. AM harvest (317 vs. 299 g/d). Time of harvest did not affect N intake, digestion, or retention. Compared to no supplement, supplement improved (P<0.01) N digestion (6.1 vs. 3.7 g/d) and retention (2.2 vs. 1.1 g/d). We conclude that PM harvest increased DM digested, largely of TNC, and digestible DM intake by goats due to increased TNC and not because of a 2% increase in intake. Adding protein supplement had very limited effects on intake and digestibility of gamagrass.