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.

Although New Zealand is fortunate in having a climate mild enough to allow some growth of pasture in winter even in the extreme south of the South Island (Duffy, 1971), growth is nevertheless considerably less than in spring and summer. O’Connor et al, (1968) point out that at Lincoln, Canterbury, winter production from a New Zealand Certified Grasslands Ruanui Perennial ryegrass (Lolium perenne c.v.) and New Zealand Certified Grasslands Huia white clover (Trifolium repens c.v.) sward is at best only 8% of the mean total annual dry matter (D.M.) production. A similar seasonal pattern is shown by irrigated pastures (Rickard, 1968), as well as pastures in the milder North Island of New Zealand (O’Connor and Vartha, pers comm.). Stocking of grazing animals is normally related to the growth rate of pastures in the spring, with conserved hay or silage, plus specially grown greenfeed or root crops being fed if necessary to offset the winter feed deficit. However on hill country too steep for tractor cultivation, cropping and conservation are not possible. Animals have to rely upon in situ grazing of native or dominant browntop (Agrostis tenuis) swards which may not have been improved by the aerial introduction of clovers. In an unimproved state, the quality and quantity of the herbage grown on such areas are low, Molloy (1966) have discussed clover introduction into native swards and the notably beneficial result this has on stock thrift, particularly during the winter period. Considerably better growth rates of stock grazing legumes rather than grasses have been noted by many authors including Ulyatt, (1971), and McLean et al (1962); this superiority being due largely to the greater digestible organic matter intake and higher ratio of soluble to insoluble carbohydrate associated with the herbage of legumes (Ulyatt, 1971). Consequently the presence of clover, and the winter activity of this, have an important influence on the productivity of steep hill country during winter. An increase of winter activity is also desirable, but not essential, in clover associated with flat or gently rolling pastures. The main pasture legume used in New Zealand has, in the past, been the white clover cultivar Huia. Whilst since 1945 selection and breeding of ryegrasses has brought about a very considerable improvement in the winter or cool season activity of these (Corkill, 1966), no legume cultivar having an increased level of cool-season activity has been released to commerce in New Zealand over the same period to date. Breeding for increased winter growth has been an objective of the Grasslands Division Plant Breeding Section for many years (Barclay, 1960). Since 1957, breeding and selection work has been proceeding with the objective of increasing the winter growth of the New Zealand Certified Grasslands white clover cultivar – Huia (henceforth to be referred to as Huia) without sacrificing the moderately good summer growth of this strain, (Barclay, 1969). Seed of a promising cultivar selected during the course of this work – New Zealand Grasslands 4700 white clover, (henceforth to be referred to as 4700) became available for testing in 1967. The investigation to be reported here aimed to measure the cool-season activity of 4700 by comparison with that of Huia, at the same time elucidating if possible the factors controlling this growth.