Intake, in vivo digestibility and protein utilization of wethers fed timothy or tall fescue when harvested at different dates in the first regrowth cycle

The objective of this study was to evaluate the effects of harvest date in the first regrowth cycle and grass species on intake, in vivo digestibility and its relation to protein utilization in wethers fed timothy or tall fescue silages. Timothy and tall fescue were harvested at regular (RTI and RTF, respectively) or late date (LTI and LTF, respectively) in the first regrowth, creating four experimental silages. Eight wethers were used in a duplicated 4 × 4 Latin square. Experimental periods lasted for 4 wk and wethers were fed ad libitum during the first 3 wk, with intake recorded during the third week. During the fourth week, wethers were fed 80% of ad libitum, and feces and urine were collected during the last 4 d. Wethers receiving RTI showed the greatest DM intake when expressed as kg/d or as percentage of body weight (BW) (P≤0.05). The intake of neutral detergent fiber (aNDFom) was affected by forage species only, where animals fed timothy silages had greater aNDFom intake than animals fed tall fescue silages (P<0.001). Intakes of CP and sum of the protein fractions A, B₁ and B₂ (AB₁B₂) were affected by the interaction between harvest date and forage species, where wethers fed RTI showed the greatest intakes of CP (P=0.001) and AB₁B₂ (P=0.02). Harvesting the forages at late date decreased the in vivo digestibility in wethers but only for timothy, where animals fed LTI silage showed the lowest DM (P<0.001), organic matter (OM) (P<0.001), aNDFom (P=0.02) and acid detergent fiber (P=0.004) digestibility, and a tendency for lower CP digestibility (P=0.07) compared with the other silages. Wethers fed RTI silage showed greater intake of nitrogen (N) (P=0.001) and digestible OM (P=0.003), greater allantoin (P=0.03) and hippuric acid (P=0.05) excretions, greater microbial N flow (P=0.03), and a tendency for greater excretion of fecal N (P=0.09) compared with the other silage-fed animals. In conclusion, delayed harvest decreased in vivo digestibility only in timothy, but even with lower in vivo digestibility wethers fed timothy silages showed a greater intake than wethers fed tall fescue silages, likely due to lower concentration of hydroxycinnamic acids observed in timothy compared with tall fescue as published previously.