Effect of date and length of closure and post grazing residual on pasture quality for silage

Abstract

The nutritional value of pasture silage made in New Zealand is often sub-optimal for lactating dairy cows. As farming systems require more pasture silage to be used in an attempt to increase lactation lengths and milksolids production per cow, the quality of this silage becomes more important. In spring 1995, pastures were closed for silage at weekly intervals from 11 September to 17 October, representing six distinct closure dates, to determine the effect that date and length of closure has on pasture quality. In spring 1997 pastures were grazed to different residuals (1.2- 1.4, 1.8-2.0, 2.1-2.4 and 2.5-2.9 t DM/ha) and then closed for silage to determine the effect that post grazing residual has on pasture quality. In both experiments pasture dry matter (DM) yield and chemical and botanical composition was monitored over the 63 day closure. Pasture closed on 11 or 17 September had higher metabolisable energy (ME) (P<0.05), organic matter digestibility (OMD) (P<0.01) and DM yields at 70% OMD than pasture closed after 9 October. Pasture closed on 11 September remained above 70% OMD for the 63 day closure while pasture closed on 17 October reached 70% OMD after only 49 days. Pasture grazed to 1.2-1.4 t DM/ha accumulated significantly less DM to 42 days post grazing than all other grazing treatments (P<0.001), but had higher crude protein (CP), OMD and ME contents (P<0.001). The low post-grazing residual resulted in pasture with more vegetative (P<0.001) and less reproductive (P<0.05) material at 42 days post grazing. These results suggest that in order to maximise the yield of pasture silage of suitable feed quality for lactating cows pasture should be last grazed before 15 September, harvested before the end of November and have a post grazing residual of approximately 1500 kg DM/ha. Keywords: closure date, closure length, herbage accumulation, organic matter digestibility, pasture quality, pasture silage, perennial ryegrass, postgrazing residual