Dairy cow performance and nutrient utilization when offered high or low digestibility grass silages at 2 levels of total diet crude protein

The hypothesis of this study was that grass silage digestibility would influence the response of dairy cows offered diets differing in CP content. Thirty-two mid-lactation Holstein dairy cows were used in a 2-period (21-d adaption phase, 7-d measurement phase), partially balanced change-over experiment. Four treatments were organized in a 2 x 2 factorial arrangement, comprising 2 grass silages differing in digestible OM in the DM (D-value; 748 and 668 g/kg DM, high-D and low-D, respectively) and 2 total diet CP levels (target 145 and 175 g/kg DM, high CP and low CP, respectively). The latter were achieved using 2 iso-energetic concentrates that differed in CP level (173 and 228 g/kg DM). Silages and concentrates were mixed and offered as a TMR in a 50:50 DM ratio. At the end of the feeding study, nutrient utilization was measured using 4 cows per treatment. Except for milk urea nitrogen there were no interactions between silage digestibility and total diet CP for cow performance and nutrient utilization. The high-D silage improved cow performance and increased nitrogen use efficiency (NUE). Reducing total diet CP also increased NUE, but there were negative impacts on cow performance. While there were benefits of offering grass silage with a high digestibility, responses to total diet CP were similar at both levels of silage digestibility.