Studies on the effects of grazing regime on sward and dairy cow performance : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University
Grazed herbage is the major source of feed for dairy cows in New Zealand. The efficiency with which herbage grown is converted into animal product is thus of major concern to the dairy industry. The objectives of the present study were to assess sward response to different grazing regimes over a range of seasons, and to measure the performance, in short term trials, of cows grazing the resultant sward types over a range of herbage dry matter allowances (DA).
Seasonal effects on sward structure and composition were great, with the effect of season often interacting with grazing regime imposed. Grazing regime had the greatest influence on sward characteristics in late spring. The cultivar mixture sown had a much less significant effect on sward characteristics measured. Swards grazed intensely and frequently had the lowest percentages of senescent matter, the highest values for herbage quality (DMD and N %) and optimal levels and patterns of NHA throughout the year.
Intense grazint or topping (I) versus lenient grazing (L) in spring resulted in those swards maintaining high percentages of grass leaf and clover, low percentages of grass stem and senescent matter and a high herbage quality into early summer. The effect of sward type on dry matter intake (DMI) in early summer was variable, but milk and milk protein yield and sometimes milk fat
yield were greater for cows grazing the I swards, at both a restricted and generous DA. When a common leaf allowance (LA) was offered, there were no significant differences in DMis or production between cows grazing the Land I swards. LA, rather than DA, was the best predictor of DMI and milk and milk protein yield over the three grazing trials conducted in early summer.
Swards that had been allowed to regrow for 60 versus 120 days in autumn/winter had greater percentages of clover, lower percentages of senescent matter and a higher herbage quality in early spring, although pre-grazing total DM masses were lower. At a common nominal DA, milk, milk fat, and milk protein yields were greater on the 60 day regrowth swards despite DMis being similar.
In all trials greater milk yields were attributed to greater percentages of grass leaf and clover and lower percentages of senescent matter and thus a higher herbage quality, both in the herbage consumed and on offer. Irrespective of sward type or DA on offer, the percentage of grass leaf was higher and the percentage of senescent matter lower, in the diet than that present in the herbage on offer.
The results of the present study emphasize the importance of maintaining swards with a high percentage of grass leaf and clover and a minimal percentage of senescent matter. This is best achieved by maintaining an intense and frequent defoliation regime, especially during reproductive growth in spring.