Management of clover in grazed pastures: expectations, limitations and opportunities

Abstract

The value of white clover as a component of New Zealand pastures is undeniable, but it is also widely recognised that clover has limitations as a pasture plant and that these can lead to inefficiencies in the performance of grass/clover associations. This paper identifies some of the limitations to optimising the contribution of clover in complex soil/pasture/animal systems, within the context of the expectations commonly held of clover. Limitations to exploiting the greater digestive efficiency and short-term intake rate of clover compared to grass when they are grown in a mixture include animal behaviour responses that sometimes impose a restriction on total daily intake of nutrients, and the fact that clover often constitutes less than 20% of the pasture. Nitrogen inputs and yield advantages are also restricted by the low clover content of pastures. A simulation model is used to analyse the co-existence of grass and clover as influenced by N dynamics. This model explains the basis for selfregulation by grass/clover mixtures of the amount of mineral N in the soil. Self-regulation minimises N losses from mixtures, but the dynamic response of grass and clover to N availability also means that there may only be limited scope for increasing the overall clover content, or decreasing the spatial heterogeneity in clover distribution, of a mixture. Managing grass/clover associations to realise the benefits of white clover therefore means manipulating a complex system, where the outcomes of manipulation depend as much on the response of the companion grass as on the response of the clover itself. Opportunities for attaining a higher clover content in pastures include: manipulating the preferences of animals for clover versus grass; spatially separating grass and clover within fields; increasing the metabolic efficiency of N fixation in clover; uncoupling the apparent link between rhizobium symbiosis and the N content of clover leaves; and modifying the stolon morphology of clover as a way of increasing clover presence in favourable microsites within the pasture. Keywords: genetic improvement, grass/clover competition, grazing behaviour, intake, models, N fixation, nitrogen dynamics, nutritive value