Effects of alternative grass species on grazing preference of sheep for white clover

Abstract

Despite the importance of a high white clover (Trifolium repens) content in temperate pastoral systems in terms of livestock performance and nitrogen fixation, the proportion of white clover in grass-clover pastures is often low (<20%). This thesis examined in two experiments whether the white clover content of pastures could be improved by sowing white clover with alternative grass species to diploid perennial ryegrass (Lolium perenne L.).

In a pasture experiment, DM production, pasture composition and morphology of grass-clover mixtures was measured over the establishment year (January 2007 to January 2008) where white clover was sown in fine mixtures with diploid perennial ryegrass, tetraploid perennial ryegrass, timothy (Phleum pratense L.) and cocksfoot (Dactylis glomerata L.). Pastures were irrigated and rotationally grazed with on-off grazing with Coopworth ewe hoggets. Total annual DM production of pasture was more than 20% higher in tetraploid (12521 kg DM ha-1) and diploid (11733 kg DM ha-1) perennial ryegrass than timothy (9751 kg DM ha-1) and cocksfoot (9654 kg DM ha-1). However, timothy (5936 kg DM ha-1) and cocksfoot (5311 kg DM ha-1) had more than four times higher white clover annual DM production than tetraploid (1310 kg DM ha-1) and diploid (818 kg DM ha-1) ryegrass. Pasture growth rate at the first three harvests in autumn was significantly greater in tetraploid and diploid ryegrass than timothy and cocksfoot. Timothy and cocksfoot had a higher proportion of white clover than tetraploid and diploid perennial ryegrass throughout the entire year. This was due to more and larger white clover plants in timothy and cocksfoot plots.

In a grazing preference experiment, the partial preference of sheep for white clover offered in combination with the same grass species as in the pasture experiment was measured in five grazing tests in May, September, October, November and December 2007. Pastures were sown in January 2007. Paired plots (grass and clover both 4.2 m x 10 m) were grazed by three Coopworth ewe hoggets between 9am and 5pm, and preference was recorded by decline in pasture mass and visual scan sampling for grazing time. Grazing preference for clover was generally low throughout these tests (e.g. average apparent DM intake from clover = 47%; average grazing time from clover = 44%). Several explanations are proposed for this low preference including a high N content and intake rate of the grass relative to the clover. No significant differences were found among the grass treatments in total grass grazing time, total clover grazing time, ruminating time, the proportion of grazing time on clover, selective coefficient for clover and DM intake percentage from clover at any date. There was no significant change in overall sward surface height (SSH) decline among grass treatments throughout all the tests except December 2007 when the overall SSH decline for cocksfoot was significantly lower than the other species.

The study indicated that the rapid growth rate of perennial ryegrass in the early phase of pasture establishment, rather than differences in partial preference, was the key factor limiting white clover content in the mixed swards relative to cocksfoot and timothy pastures. It is concluded that high clover-containing pastures capable of delivering high per head performance can be established through the use of slow establishing pasture species such as timothy and cocksfoot.