Effect of stage of growth at harvest and level of urea application on chemical changes during storage of whole-crop wheat

Winter wheat (Triticum aestivum L.) harvested at a range of stages of growth has become a popular conserved forage for winter feeding of dairy cattle. The detailed processes of the preservation of whole-crop wheat were studied in three experiments in 1989 (Experiment 1) and in 1990 (Experiments 2 and 3). A range of maturities of wheat (growth stages 49, 71 and 87 in Experiment 1 and growth stages 53, 70, 80, 83 and 85 in Experiment 2) were selected and ensiled with three (0, 20 and 40 kg (t DM)(-1); Experiment 1) or four rates (0, 20, 40 and 60 kg (t DM)(-1); Experiment 2) of application of urea in small-scale silos. The experiments were based on factorial designs with crop maturity (oven dry matter (DM) content) and level of application of urea as the principal factors. Experiment 3 considered the changes in chemical composition over time (silos opened after 0, 10, 20, 30, 60 and 90 days ensiling) at two level of urea treatment (0 and 40 kg (t DM)(-1)) and two stages of growth (GS 70 and 83). The rate of hydrolysis of urea was dependent on the moisture content of the ensiled forage and the level of urea applied (Experiment 3). Microbial activity, measured by the production of total fermentation acids (lactic acid+VFA), was progressively reduced in both experiments by the level of urea applied (p<0.001; Experiments I and 2) and increasing maturity of forage at harvest (p<0.001; Experiments 1 and 2). An interaction (p<0.001; Experiments 1 and 2) was observed for production of total fermentation acids between level of urea applied and maturity of forage, reflecting the relative availability of nutrients, in particular sugars (WSC)) for microbial utilization. Losses of DM during ensiling were progressively reduced (p<0.001; Experiments 1, 2 and 3) by the addition of urea at ensiling and by increasing maturity of the forage (p<0.001; Experiments 1 and 2) and duration of storage (Experiment 3). Ensiling the forage without urea reduced its NDF content (p<0.01; Experiment 1 and p<0.001, Experiment 2), but with increasing crop maturity the extent of the reduction in NDF was decreased. Application of urea at a rate >40 kg(t DM)(-1) increased (p<0.01; Experiments 1 and 2) the reduction in NDF content compared to forage ensiled without urea. Treatment of harvested forage at ensiling with urea reduced (p<0.001: Experiments I and 2) the conversion of WSC to fermentation acids during conservation but slightly enhanced the loss of starch (p<0.05: Experiment 2). There was a marked reduction (ranging from 6 to 17 g kg(-1) (p<0.01); Experiments 1 and 2 to 23 g kg(-1) (p<0.01); Experiment 2) in the digestibility of the ensiled forage compared to fresh forage, dependent on maturity of the forage. but no differences in digestibility were observed between urea treated forage and fresh forage. Treatment of whole-crop wheat forage with urea at harvest can be considered an effective method of preserving forage nutritive value, (C) 1999 Elsevier Science B.V. All rights reserved.