Effects of total nonstructural carbohydrates and nitrogen balance on voluntary intake of goats and digestibility of gamagrass hay harvested at sunrise and sunset

摘要

We evaluated differences in composition of Iuka gamagrass (Tripsacum dactyloides L.) hay harvested at 06:00 (AM harvest) or 18:00h (PM harvest), and measured how protein supplementation and time of harvest interact to affect the voluntary intake, digestibility, and N balance of goats. Boer cross wethers (n =28; 24kg) were randomly assigned to be fed supplement (310g/kg of crude protein (CP), fed at 110g/kg of dry matter (DM) intake, 14 goats) or no supplement (14 goats). Within supplemented or notsupplemented groups, goats were randomly assigned to a crossover design of AM harvest (seven goats) or PM harvest (seven goats), and housed individually in metabolism crates with free access to water and mineral blocks. They were fed twice daily, with supplement offered 30min prior to the morning feedings. After a 7-d adaptation, voluntary intake (goats were offered 1100g/kg of previous day's intake) was measured for 14d, followed by a 4-d adjustment phase to equalize DM offered between periods, and finally a 5-d digestion and balance phase. After Period 1, goats were switched to their new hay harvest times, and the protocol was repeated. Compared to the AM harvest, the PM harvest had higher (P<0.03) proportions of total nonstructural carbohydrates (TNC, 70.8g/kg DM versus 59.0g/kg DM), monosaccharides (37.0g/kg DM versus 28.6g/kg DM), di- and polysaccharides (18.5g/kg DM versus 15.4g/kg DM) and less neutral detergent fiber (NDF, 700g/kg versus 710g/kg). Crude protein (79g/kg DM) and starch (15.2g/kgDM) were similar for the PM and AM harvest. Dry matter digestibility was higher (P<0.03) for the PM versus AM harvests (555g/kg DM versus 531g/kg DM) and for supplemented versus not supplemented (563g/kg DM versus 522g/kg DM). Voluntary gamagrass DM intake (550g/d versus 548g/d) and calculated total digestible DM intake (327g/d versus 313g/d) were similar for the PM and AM harvest. However, total digestible DM intake during the digestion and balance phase was higher (P<0.01) for the PM versus AM harvest(317g/d versus 299g/d). Time of harvest did not affect N intake, digestion, or calculated retention. Compared to no supplementation, the supplement improved (P<0.01) N digestion (6.1g/d versus 3.7g/d) and retention (2.2g/d versus 1.1g/d). The PM harvestincreased DM digested, largely TNC and digestible DM intake by goats due to increased TNC and not because of a 2% increase in DM intake. Providing a protein supplement had very limited effects on intake and digestibility of gamagrass.