Influence of nitrogen fertilization and defoliation frequency on nitrogen constituents and feeding value of annual ryegrass.

Two experiments were conducted to determine the seasonal effects of N fertilizer (NF) and defoliation interval (DI) on specific N fractions within annual ryegrass (AR) and overall usage of AR N by growing lambs. In Exp. 1, NF (0, 112, 224, and 336 kg of N/ha) and DI of 2, 4, or 6 wk were factorially applied to AR plots. Both decreasing DI and increasing NF increased N levels in AR in a manner that depended on harvest date (NF or DI x harvest date interaction, P < .05). The proportions of .2% NaOH soluble N or soluble protein N were not affected (P > .05) by NF or DI. Although NF had no effect (P > .05) on in situ disappearance of DM, decreasing the DI seemed to reduce the severity of the season-induced reduction in DM disappearance (DI x harvest date interaction, P = .04). In Exp. 2, NF (0 or 224 kg of N/ha) and a DI of 2 or 6 wk were factorially applied to AR plots. Fresh, frozen AR was fed in semipurified diets (48% of DM as AR) to four wether lambs (23 +/- 1.3 kg) using a 4 x 4 Latin square. Diets including AR had greater ruminal DM and plant N digestibilities when AR had a 2-wk vs a 6-wk DI with NF but had reduced digestibilities when AR had a 2-wk vs a 6-wk DI without NF.(ABSTRACT TRUNCATED AT 250 WORDS)