Performance of dairy cows offered silages produced from grass swards harvested on either 3 or 5 occasions during the growing season.

The nutritive value of grass silage can be improved by harvesting herbage at a less mature growth stage, which in practice usually involves more frequent harvests. This study examined the performance of dairy cows offered grass silages produced from perennial ryegrass (Lolium perenne) based swards harvested at 2 different frequencies during the growing season (3-harvest [3H] vs. 5-harvest [5H]). Thirty-four mid-lactation (average 147 DIM) dairy cows (30 multiparous, 4 primiparous) were offered either 3H or 5H silages in a continuous design (21 wk) experiment. Within each treatment cows were offered silage from each harvest (in harvest number order) for a predetermined number of days in proportion to herbage DM yield at each harvest. Silages were offered ad libitum, while a common concentrate was offered to all cows at 12.0 kg/cow per day over the first 15 wk of the study, 8.0 kg/cow per day during wk 16 to 19 and 6.0 kg/cow per day during wk 20 to 21. Total yield of herbage harvested over the season from within 3H and 5H were 12.6 and 11.2 t DM/ha, respectively. Across all harvests the mean ME and CP concentration of silages were 10.9 MJ/kg DM and 131 g/kg DM for 3H, and 11.5 MJ/kg DM and 152 g/kg DM for 5H. Silage DMI was greater for cows offered 5H silages compared with 3H silages (14.1 vs. 11.7 kg/d, respectively). Cows offered 5H silages had a greater daily milk yield (33.5 vs. 31.9 kg) and ECM yield (37.4 vs. 35.6 kg) compared with cows offered 3H silages. Treatment had no effect on milk fat or protein concentration. Cows offered 5H silages produced milk with greater concentrations of CLA and n-3 fatty acids. Treatment had no effect on mean BW or BCS, or on efficiency metrics such as milk yield or ECM yield per kilogram of DMI. Molar proportions of VFA in ruminal fluid differed between the treatments, with cows offered 3H silages having higher proportion of total butyrate (15.9% vs. 14.4% of total VFA) and lower total valerate (3.2% vs. 3.7% of total VFA) compared with those offered 5H. The acetate:propionate and acetate plus butyrate:propionate ratios were unaffected by treatment. In conclusion, increasing herbage harvesting frequency from 3 to 5 times per year improved the nutritional value of the resulting silages, and this led to higher silage DMI, milk yield, and ECM yield. However, overall production efficiency (ECM/DMI) was unaffected by treatment.