A three-year investigation of the performance of spring-calving dairy cows grazing ryegrass-based pastures of high or low endophyte toxin status in Northland

Abstract

On-farm milk production and ancillary information was monitored for spring-calving cows grazing high (HT) and low toxin (LT) pastures in Northland over three seasons from spring 1997 to autumn 2000. Two groups of 16 second-calving Holstein-Friesian cows, balanced for calving date and production worth, grazed HT or LT ryegrass pastures from October 1997 to May 1998. Milk production was monitored for 10 consecutive days each month culminating with a herd test. The LT group out-produced the HT group for each measurement period from December, and over the whole season, produced 20% more milk solids. The HT group had significantly higher serum lysergol levels and correspondingly lower prolactin levels than the LT group. As pasture quality was similar it was suggested that the differences in milk production were most likely due to differences in intake. At the start of the 1998-1999 season 25% of the farm was in low endophyte ryegrass or in ryegrass with the AR1 endophyte. The farm and herd were split so that milk production from HT and LT farmlets could be monitored. Milk from each group was collected into separate tanks and all cows were 'herd-tested' 4-weekly. Despite twothirds of the LT farmlet being conserved for silage, which resulted in a severe feed shortage that necessitated supplements being fed-out on this farmlet from January, the LT group out-produced the HT group by 5% per cow and by 10% per hectare for the season from October to late May. In the 1999-2000 season, the farm and herd split was 29% LT and 71% HT. The objective was changed to one of maximising production from each system and this required changes in management. The LT system was managed to encourage clover-dominance from mid-spring and this was achieved by lengthening the rotation out to 6 weeks and then reducing it back to 3.5 weeks by leaving higher residuals and conserving <10% of the surplus. In contrast, a fast rotation (2.5 to 3 week) was used for the HT system and surplus feed was conserved. The LT pastures contained considerably more clover than the HT pastures. The only differences in pasture quality were in the November-December period when protein and soluble carbohydrate levels were higher in LT than HT pastures. The LT system out-produced the HT system by 8% per cow and by 12% per hectare. Issues surrounding the establishment and maintenance of LT pastures are discussed. Keywords: dairy cows, endophyte toxins, milk production, Northland, ryegrass