"Buying a spring" - the water and nitrogen cost of poor fallow weed control

Research in southern NSW demonstrated that 50% of yield potential can be attributed to summer rainfall and summer fallow management as a result of increased stored water (increased by 49 mm) and nitrogen (N) (increased by 49 kgN/ha). This paper reports experiments conducted near Forbes in central NSW (2011 & 2012) to further evaluate the impact of summer weed control on subsequent crop yield, to investigate interactions with other macro-nutrients (P, K and S) and assess the profitability of replacing lost N (via summer weed uptake) with N fertiliser. Controlling summer weeds increased canola grain yield by 1.0 t/ ha due to increased stored water (85 mm PAW) and mineral N (69 kgN/ha) at sowing. For every 1mm of stored water stored through summer weed control, soil mineral N increased by 0.6 kgN/ha. Summer weeds had no significant impact on topsoil P and K levels, or S to a depth of 90cm. Every $/ha invested in fallow herbicides returned $8/ha. Despite poor topdressing conditions, profitability of applying N fertiliser improved from $1 return on every $1 invested when weeds were not controlled to $3 return on every $1 invested in the complete weed control treatment. The study demonstrates the value of strict summer weed control to improve productivity and resource-use efficiency in southern cropping systems.