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

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:l) 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.