Performance of dairy cows offered either zero-grazed grass or grass silage prepared from the same sward

Daily harvesting of fresh grass for housed livestock when its nutritive value is high (Zero-grazing) is a labour-intensive process which requires a consistent supply of grass at the optimum growth stage. An alternative approach which may save on labour and require less time spent on grassland management each day, involves harvesting and ensiling herbage on a number of occasions (every 4 weeks approximately) during the growing season when it is at the same nutritive value as herbage used for zero-grazing. This study examined the impact of these two approaches to dairy cow performance. Thirty-six mid-lactation Holstein-Friesian dairy cows were offered either zero-grazed fresh grass (ZG), or grass silage (SIL) prepared from the same sward harvested at a similar growth stage, over a single season. Fresh grass was harvested daily and offered to ZG cows for a 12-week period. During this period, the same sward was harvested once weekly and ensiled in round bales. Following a five-week ensilage period, the silage was offered to cows on SIL for a 12-week period. All cows were also offered 8.0 kg concentrate per day. Zero-grazed grass and grass silage had a mean metabolisable energy content of 11.0 and 11.3 MJ/kg DM, respectively. Mean forage DM intake (DMI) and total DMI were greater for cows on ZG (P < 0.001) compared to SIL, with intakes during weeks 8-12 of the experiment lower with SIL compared to ZG (P < 0.001). Cows on ZG had a higher milk yield, milk protein concentration, milk fat plus protein yield and energy-corrected milk (ECM) yield (P < 0.001) than cows on SIL. With the exception of milk yield (where the difference was primarily observed during weeks 8-12 of lactation), these differences were observed most weeks during the study period. Milk fat concentration was unaffected by treatment (P > 0.05). Milk of cows on ZG had higher concentrations of total monounsaturated fatty acids (P < 0.001), total polyunsaturated fatty acids (PUFAs, P < 0.001) but lower concentrations of saturated fatty acids (SFAs, P < 0.001) compared to cows on SIL. Diet had no effect on cow BW or condition score. This study has shown that when harvested from the same sward, milk yield and ECM yield were improved when cows were offered zero-grazed grass compared to grass silage. This difference in performance was likely due to the lower forage intake observed with the grass silage-based diet.