Prospects for improving productivity and composition in pasture/crop rotations by changing row configuration at sowing

The challenge of maintaining a diverse and robust mixture of pasture species in the rain‐fed, semi‐arid environments common across south‐eastern Australia has long been a topic of interest to farmers and researchers alike. The competition for scarce water and nutrient resources, particularly during summer, is often a key factor driving the decline of desirable species over a relatively short timeframe. Managing interspecific competition by changing the spatial arrangement of different pasture species at sowing could be a practical solution that requires little increase in expense or management complexity for the farmer. However, few studies have investigated the effectiveness of this strategy in improving the productivity of pasture swards, nor examined the broader implications of this approach to farming systems where pastures are grown in phased rotations with crops. This thesis draws on data from a network of field sites comprising two series of experiments. The first, located in the Riverina region of southern New South Wales (NSW), Australia, compared ten pasture swards containing lucerne (alfalfa; Medicago sativa L.), phalaris (Phalaris aquatica L.) and/or subterranean clover (Trifolium subterraneum L.) in various drill row configurations. The second, located in the Central West region of NSW compared legume establishment under cover crops of wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), canola (Brassica napus L.) and lupin (Lupinus angustifolius L.) sown in mixed and alternate drill row configurations with lucerne and subterranean clover. These investigations aimed to evaluate the impact of changing row configuration at sowing on pasture productivity and persistence, and examine the legacy of this practice on soil fertility, the microbiome as well as subsequent crop production. Sowing species into fewer drill rows often led to reduced overall biomass, although results varied from site to site with differences less evident under drier seasonal conditions. Less total biomass was attributable to a reduction in the productivity of the dominant species in the mixture. Herein lies the ultimate challenge when managing competition in a mixture by increasing the spatial distance between species at sowing. On the one hand the spatial separation of species reduced early competition to improve the opportunity for less vigorous species to establish. On the other hand, constraining the dominant species to fewer drill rows limited the productivity of that species, which frequently resulted in reduced overall productivity of the sward. This was because constraining species to fewer drill rows effectively increased the spacing between those drill rows, and the yield of companion species in the intervening rows often could not compensate for the yield reduction of the dominant species. Nowhere was this seen more clearly than in a covercrop scenario where the yield reduction associated with confining the vigorous winter crop to alternate drill rows was not matched by the production of slow‐establishing pasture legume seedlings in every other drill row. The legacy of row configuration on perennial‐based pasture productivity is enduring. All three pasture species tested were observed to remain largely confined to the original drill for the duration of the 3‐year pasture phase, although a greater proportion of the self‐regenerating subterranean clover was found on the inter‐row area (up to 30%) compared to the perennial species (<5%). This was less evident at the hotter and drier sites where