Nutrients in ryegrass (Lolium spp.), white clover (Trifolium repens) and kikuyu (Pennisetum clandestinum) pastures in relation to season and stage of regrowth in a subtropical environment

Abstract

Summary. On the subtropical north coast of New South Wales, Australia, kikuyu grass (Pennisetum clandestinum), biennial ryegrass (Lolium multiflorum) and mixed perennial ryegrass (Lolium perenne)–white clover (Trifolium repens) pastures grazed by dairy cows were plucked pregrazing to simulated grazing height, every 2 weeks for 2 years to determine seasonal changes in various nutrients and in sacco organic matter and nitrogen (N) degradability. Changes in nutrients during regrowth were determined in the ryegrass component of a mixed perennial ryegrass–white clover pasture by sequentially cutting pasture at 3- or 4-day intervals to 5 cm stubble height and non-structural carbohydrates in kikuyu by cutting at 4-day intervals in February–March. There was a significant effect of season on water-soluble carbohydrate (WSC) and crude protein (CP) content of perennial ryegrass with regrowth time, resulting in an 8-fold fall in the CP : WSC ratio from the 1 to 3 leaves/tiller stage of regrowth in mid winter, a 2-fold difference in mid spring but with no discernible difference in late spring. The metabolisable energy (ME) values for biennial ryegrass exceeded 11.9 MJ/kg dry matter (DM) from July to September and then fell markedly to <10 MJ/kg DM in November, coinciding with reproductive development. In perennial ryegrass–white clover pastures, mean ME was above 11 MJ/kg DM from May to September, but fell to < 9 MJ/kg DM in December while in kikuyu, the mean ME, over the recognised growing season, was 8.5 MJ/kg DM but in winter it was 9.5 MJ/kg DM. Fibre content in all pasture types showed a significant seasonal trend with the content of acid detergent fibre (ADF) in biennial ryegrass at 17% from May to August while the mean neutral detergent fibre (NDF) content was 37%. In perennial ryegrass–white clover, the mean ADF was <21% from May to August. The NDF content of kikuyu grass was about 60% during the growing season but 40% in winter. The calcium (Ca) : phosphorus (P) ratio in perennial ryegrass rose from <1 : 1 at the 1 leaf/tiller stage to 2.2 : 1 at the 3 leaves/tiller stage of regrowth due to a simultaneous fall in P and a rise in Ca. A fall in potassium (K) and a rise in magnesium (Mg) and Ca content in perennial ryegrass gave a very significant linear fall in K/(Mg + Ca), on a percentage basis, from 8 at the 1 leaf/tiller stage of regrowth, to 2.5 at the 3 leaves/tiller stage of regrowth. In kikuyu, the level of P changed significantly with season falling as the species became dormant. A fall in P and a rise in Ca content resulted in a high Ca : P ratio (2.5 : 1) in spring. The findings of this study give some insight into the reason why the content of various nutrients change in pasture and the implication of this for providing a balanced diet to dairy cows. A knowledge of these changes should provide the opportunity to balance nutrients in pasture by adjusting time of grazing and/or providing supplements of appropriate quality.