Nilantha R. Hulugalle1,2, Benjamin J. Lenehan3,4, Guna Nachimuthu1 and Daniel KY Tan3
1Australian Cotton Research Institute, New South Wales Department of Primary Industries, Narrabri, NSW, Australia,
2 Present address: Fenner School of Environment and Society, Australian National University, Canberra, ACT, Australia,
3 The University of Sydney, Sydney Institute of Agriculture, School of Life and Environmental Sciences, Faculty of Science, Sydney, NSW, Australia
4 Present address: Delta Agribusiness, Harden, NSW, Australia, Email: email@example.com
Wheat is commonly sown in rotation with cotton in Australian cotton farming systems. Uncontrolled weed growth can inhibit wheat growth and thereby, have a detrimental effect on the following cotton crop. Weeds that are frequently found in wheat crops of north-western New South Wales (NSW) include deadnettle (Lamium amplexicaule L.), sow thistle (Sonchus oleraceus L.), and annual Phalaris (Phalaris paradoxa L.). The objective of our study was to quantify the effects of wheat sowing rates on weed populations and growth, and their impact on wheat growth and yield. An experiment that consisted of wheat sowing rates of 0 (fallow), 30, 60, 120 and 180 kg ha-1 was conducted during 2014 and 2015. Weed biomass, type (broad-leaved or grassy), and species diversity were assessed at wheat anthesis. Wheat phenological events (emergence, anthesis, maturity dates) were recorded and wheat biomass measured at 10 weeks after sowing (2015) and anthesis (2014, 2015), as well as grain yield. Weed populations at wheat anthesis consisted of broad-leaved weeds during 2014 and a mix of broad-leaved and grassy (annual Phalaris) weeds during 2015. Weed biomass decreased sharply when wheat was sown, even at the lowest sowing rate of 30 kg ha-1 and continued to decrease such that at the higher sowing rates it was negligible. Adequate yields can be attained by sowing wheat at rates in the range 30-60 kg ha-1. The costs associated with sowing at higher rates cannot be justified, although significant long-term reductions in the weed seedbank are likely only by sowing at these rates.