The effect of stocking rate and calving date on grass production, utilization and nutritive value of the sward during the grazing season

A 2-year whole-farm system study compared the accumulation, utilization and nutritive value of grass in spring-calving grass-based systems differing in stocking rate (SR) and calving date (CD). Six treatments (systems) were compared over two complete grazing seasons. Stocking rates used in the study were low (25cowsha(-1)), medium (2.9 cows ha(-1)) and high (3.3 cows ha(-1)), respectively, and mean CDs were 12 February (early) and 25 February (late). Each system had its own farmlet of eighteen paddocks and one herd that remained on the same farmlet area for the duration of the study. Stocking rate had a small effect on total herbage accumulation (11 860 kg DMha(-1) year(-1)), but had no effect on total herbage utilization (11 700 kg DM ha(-1) year(-1)). Milk and milk solids (MS; fat+protein) production per ha increased by 2580 and 196 kg ha(-1) as SR increased from 2.5 to 3.3 cows ha(-1). Milk production per ha and net herbage accumulation and utilization were unaffected by CD. Winter feed production was reduced as SR increased. Increased SR, associated with increased grazing severity, resulted in swards of increased leaf content and nutritive value. The results indicate that, although associated with increased milk production per ha, grazed grass utilization and improved sward nutritive value, the potential benefits of increased SR on Irish dairy farms can only be realized if the average level of herbage production and utilization is increased.