The effects of processing at harvest and cutting height of urea-treated whole-crop wheat on performance and digestibility in dairy cows

Urea-treated whole-crop wheat harvested at growth stage (GS) 87 that was either unprocessed (U) or processed (P) by the forage harvester, and cut at one of two heights to produce a long straw (L), or short straw (S) forage, was offered to 44 Holstein-Friesian dairy cows for 14 weeks according to a 2 x 2 factorial design. Whole-crop wheat (WCW) was mixed 2 : 1 on a dry-matter (DM) basis with grass silage and all animals received 8.5 kg/day of concentrate and 2 kg/day of rapeseed meal. Increasing the cutting height at harvest increased the starch content in the forage from an average of 356 to 419 g/kg DM and decreased neutral-detergent fibre levels from 422 to 337 g/kg DM. Forage intake (grass silage and WCW) was higher in cows given the unprocessed compared with the processed forages (13.7 v. 12.5 kg DM per day for treatments U v. P respectively; P < 0.05) and tended (P < 0.08) to be higher in cows given the long compared with the short straw forages. Neither processing, nor alteration of cutting height, had a significant effect on milk yield (kg/day). However, milk fat content decreased (P < 0.05) with increasing cutting height (41.9 v. 37.0 g/kg for the main effects of L v. S respectively) whilst body condition score was lower (P < 0.05) in cows given the long compared with the short straw forages (2.6 v. 2.8 respectively). Plasma glucose levels were higher (P < 0.05) in animals receiving the short compared with long straw forages (3.52 v. 3.34 mmol/l respectively) whereas β-hydroxybutyrate decreased (P < 0.01) with increased cutting height. The average digestibility of starch in the total ration was higher (P < 0.001) in cows given the processed compared with the unprocessed WCW (0.96 v. 0.88 respectively). Assuming a constant apparent digestibility of starch in the other food components resulted in a WCW starch apparent digestibility of 0.87, 0.97, 0.80 and 0.96 for forages LU, LP, SU and SP respectively (P < 0.001). The results indicate that processing at harvest significantly increased the digestibility of starch in WCW and that cows responded to this higher nutrient content by decreasing forage intake. Increasing the cutting height at harvest decreased milk fat content (g/kg) and yield (kg/day) and increased body condition score.