Growth and milksolids production in pastures of older and more recent ryegrass and white clover cultivars under dairy grazing

Dairy milksolids (MS) production rates were measured in pastures consisting of different age-classes of perennial ryegrass and white clover cultivars. There were four pasture treatments consisting of factorial combinations of ryegrasses representative of those available to fanners in the 1980s and 1998, and clovers available in the 1960s and 1998. The pasture treatments were grown in self-contained, replicated 4 ha farmlets and rotationally grazed at a stocking rate of three Friesian cows per hectare. The 1998 ryegrass and clover cultivars were more competitive than the older cultivar types, and tended to form a higher proportion of the sward at the end of the trial. This did not translate into significant cultivar age group effects on short-term pasture growth rates or annual pasture dry matter (DM) yields which averaged 17.2 +/- 0.9 t DM/ha over years 2-4 of the trial. The 1980s ryegrasses had faster growth rates in spring than the 1998 ryegrasses. Consequently, more silage was made on average on farmlets sown with the 1980s ryegrasses (230 kg silage DM/cow per yr), than with the 1998 ryegrasses (150 kg silage DM/cow per yr), although the yield increase was statistically significant only in the 2001-02 year. Herbage chemical profiles and feeding quality were similar in all treatments. Nitrogen fixed per unit clover grown did not vary with clover types. There was a very strong linear correlation between annual clover yields and annual N fixation. Annual N fixation totals varied substantially from the mean value of 157 kg N/ha., as clover annual DM yields changed. Total annual milksolids (MS) production ranged from 811 kg MS/ha per yr to 1250 kg MS/ha per yr, with no consistent pasture treatment effects. There was no significant effect of ryegrass type on milk production per cow or milksolids production per hectare in any year. Clover type had an effect on production only in 2001/02, with higher milk production per cow and total milksolids production per hectare on the 19918 clover treatments, due to the higher clover content. of pastures in this treatment. We suggest that annual pasture production from these well-managed ryegrass-white clover pastures is very close to the practical limit achievable in this region. Any major increases in herbage yield will require novel plant germplasm.