Prospects for improving productivity and composition of mixed swards in semi-arid environments by separating species in drill rows – A review

Establishing multi-species swards is a common objective in agricultural forage production systems around the world. Managing pasture productivity and sward resilience is especially challenging in semi-arid environments where mixtures of perennial and self-regenerating annual species are expected to co-exist while growing in competition under water-limited conditions. This review explores the implications of competition dynamics and the practice of changing row configuration at sowing as a prospective approach to managing the composition and productivity of multi-species swards. It uses three test case species representing different functional groups, including the perennial legume, lucerne (alfalfa; Medicago sativa L.), the perennial grass, phalaris (harding grass; Phalaris aquatica L.) and the self-regenerating annual legume, subterranean clover (Trifolium subterraneum L.). Early field studies showed consistent benefits of separating subterranean clover from other sward components at sowing, in terms of foliage dry matter and regeneration density of that species. However, total sward productivity either remained unchanged or declined compared to where all species were planted in the same drill row, especially where forage species were sown with grain cover crops. This is explained by a basic principle of ecology where dominant species drive total production in a short-term ecosystem such as a phased pasture. Constraining dominant species to fewer drill rows at sowing limited their production due to a transient restriction in resources, such as light. It is concluded that separating species in drill rows is a useful strategy for improving the abundance of transient or subordinate species when grown in mixtures with dominant species. However, to avoid perverse outcomes, care is required to ensure that the productivity of the dominant species is not compromised. Practical implications point towards maintaining the number of drill rows to which dominant species are sown. Further research is required to minimise drill row spacing, thus maximising the number of drill rows and increasing early plant coverage on a given area of land. The legacy effect of the pasture drill row is also highlighted, with most species remaining close to the original drill row for the life of a 3-year pasture phase in semi-arid environments. Increased concentration of plants led to enduring benefits in soil fertility, microbial diversity and abundance compared to the inter-row area, findings that warrant closer examination under a greater range of soil and climatic conditions.