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Summer spraying is rarely at the top of a farmer’s favourite job list. Waking up during the early hours of the morning, new restrictions on the use of 2,4-D, increased spray record keeping and the added chemical and operational costs, all add to the chore of summer spraying. Most of us understand the benefits of controlling summer weeds, but what economic benefit can we expect from maintaining clean paddocks throughout the summer months?

Although there are many factors that contribute to a positive economic impact from summer weed control, the two greatest contributors are:

  1. Increasing the PAW (Plant Available Water): Water stored deep in the soil profile usually has a much higher value due to a higher water use efficiency (WUE) (kg grain/mm plant available water). Deep water has a higher WUE as it is more likely to assist during grain number determination and grain fill than water stored closer to the surface (Cameron, J. and Storrie, A. (eds). (2014). Much research has been done to understand the increase in seasonal WUE. According to Dr James Hunt water use efficiency increased by 60% on average where summer weeds were controlled (Madelyne McGrath 2015).
  2. Preserving mineral nitrogen that will be available to the following crop. General guidelines are that for every 1mm of moisture lost from summer weed growth mineral nitrogen levels are depleted by approximately 0.64 kg N/ha (McMaster, 2013). Research by the BCG in 2012 showed an extra 48kg/ha of N available to the following crop with appropriate weed control. Depending on Urea price this would equate to $36 – 52/ha of extra nitrogen.

Research conducted over a five-year period concluded that conserving summer fallow moisture can increase the yield and profit margin of a crop by $155/ha with appropriate weed control (BCG 2012 Season Research Results).

Trials carried out at Curyo in 2008 showed that wheat yields doubled (1.2 t/ha to 2.5t/ha) under the full program of summer weed control. At present pricing of $350/t for APW this would equate to an extra $525/ha of income. The table below puts some economic analysis around that trial.

Level of Control


Cost of Operation



(APW $300/T)

Unsprayed Fallow

$0 1.2 t/ha 360
Two spray – Full control 2 x $25/ha (Chemical and Operational costs)

= $50

2.5 t/ha


If we assume that the crop in the above scenario had a water use efficiency of 20 kg/mm rainfall, then the crop had access to an additional 65mm of PAW. Fallow efficiency of summer rainfall can range from 0-60% of rainfall depending on management practices (Stubble retention, soil type, tillage) with the normal range being 20-30%.  Many areas received good rains in Summer 2018/19, with Birchip being one of the best at 200mm plus of rain in mid-December 2018. If weeds were controlled, then fallow efficiency of at least 25% is possible and results in 50mm of additional PAW. This would equate to a possible 1t/ha (20 kg/mm WUE) yield benefit for cereals.

Good stored moisture increases the probability of profits from crops such as canola and legumes, hence 2019 crop rotations are likely to include more canola and legumes where stored moisture is above 50mm. Stored moisture is a strong driver of yield potential however it’s not the full story so further assessment of the seasonal outlook will be beneficial prior to seeding these higher risk/higher cost crops.

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