Effects of dryland summer cover crops and a weedy fallow on soil water, disease levels, wheat growth and grain yield in a Mediterranean-type environment

Numerous studies have reported wide-ranging benefits of cultivating cover crops during the typical fallow period in temperate cropping systems. However, data on the impacts of summer cover crops in water-limited, Mediterranean-type climates, is lacking. A 3-year field experiment was conducted near the town of Northam in the wheat-belt of Western Australia to test the hypothesis that inclusion of summer cover crops would increase ground cover, water storage and grain yield in subsequent wheat crops. White French millet (Panicum miliaceum) and cowpea (Vigna unguiculata) cover crops and a weedy fallow were compared to a typical chemical fallow over the summer period. Autumn groundcover was 10–30% higher when summer weeds were left uncontrolled compared to the chemical fallow treatment. Dry-sown and late established (February) millet failed to increase groundcover, whereas cowpea demonstrated a small increase (~9–10%) in one season, when cowpea biomass approached 1 t ha−1. Cover crops and weeds growing over summer had no significant effect on total plant available water (0–50 cm depth) prior to sowing wheat in any season. However, there was less readily available soil N in the cover crop (less 10–30 kg N ha−1) and weedy fallow (less 50 kg N ha−1) treatments prior to the second wheat crop. Despite some significant effects of weedy fallow and cowpeas on Gaeumannomyces graminis var tritici (take-all) and Didymella pinodes (causing black pea spot), disease levels were low and thought to pose little risk to following winter crops. Wheat grain yields were not significantly different between treatments, other than a low yield after cowpeas in 2015 due to a low wheat emergence count. Grain protein and screenings were not generally affected by summer fallow treatment in any year, except for higher screenings after the weedy fallow in one year. Given the limited impacts of cover crops and weeds on soil water and disease, it is possible that summer cover crops may be able to be integrated with cereal winter rotations in Western Australia without yield reductions in wheat crops. In light of current chemical usage, issues around herbicide residues in soil and herbicide-resistant weeds, it would be worthwhile revisiting the need for chemical fallow, particularly in the higher rainfall areas.