Effects of concentrate crude protein content on nutrient digestibility, energy utilization, and methane emissions in lactating dairy cows fed fresh-cut perennial grass

Although many studies have investigated mitigation strategies for methane (CH4) output from dairy cows fed a wide variety of diets, research on effects of concentrate crude protein (CP) content on CH4 emissions from dairy cows offered fresh grass is limited. The present study was therefore designed to evaluate effects of cow genotype and concentrate CP level on nutrient digestibility, energy utilization and CH4 emissions of dairy cows offered fresh grass based diets. Twelve multiparous lactating dairy cows (6 Holstein and 6 Holstein × Swedish Red) were blocked into 3 groups within each breed and assigned to low, medium or high CP concentrate diet (14.1, 16.1 and 18.1 % on dry matter (DM) basis), respectively, in a 3-period changeover study (25-d / period). Total diets contained (DM basis) 32.8 % concentrates and 67.2 % perennial ryegrass, which was harvested daily. All measurements were undertaken during the final 6-d of each period; digestibility measurements for 6-d and calorimetric measurements in respiration chambers for 3-d. Feed intake and milk production data were reported in a previous paper. No significant interaction between concentrate CP level and cow genotype on any parameter was observed. Concentrate CP level had no significant effect on any energy utilization parameter, except for urinary energy output which was positively related to concentrate CP level. Similarly concentrate CP content had no effect on CH4 emission (g/d), CH4 per kg feed intake or nutrient digestibility. The crossbreeding of Holstein cows significantly reduced gross energy, digestible energy and metabolizable energy intake, heat production and milk energy output. However, cow genotype had no significant effects on energy utilization efficiency or CH4 parameters. Furthermore, the present study yielded a value for gross energy lost as CH4 (5.6 %) on fresh grass-based diets that is lower than the widely accepted value of 6.5 %. The present findings indicate reducing concentrate CP content from 18.1 to 14.1 % may not be a successful approach to alleviate CH4 emissions from lactating dairy cows offered good quality fresh grass, however grazing cows could be offered a low CP concentrate without compromising energy utilization efficiency. Further research is needed to investigate whether larger differences in dietary CP content may yield positive results.