Summer fallow moisture and nitrogen loss due to weeds...

With Steve Muller



Fallow weed control has been a common farming practice for quite some time now through use of herbicides, cultivation or grazing.

The main aim has always been the same - to control weeds, however timing of weed control can be difficult, and at the moment given the current weather conditions, sometimes the decision to control weeds can be quite tough. Paddocks with very low weed populations and weeds that are impacted by drought stress, often lead us to take a more relaxed outlook on their control. With this in mind I think it is a timely reminder to talk about the potential impact of weeds on soil moisture and nitrogen levels.

Research has shown that early and effective weed control can increase potential crop yields and quality through increases in plant water and nutrient availability (in particular nitrogen [N]). This proactive approach allows for better risk management and a more stable income. The stored water saved by early weed control is usually located at depth, and this is often the water that the crop uses late in dry season to maintain grain number during the critical period of stem elongation to flowering. In certain soil types, deep soil water can have a water use efficiency of up to 60kg of grain/mm of stored water (Cameron & Storrie, 2014).

In 2010-12 the GRDC funded a series of trials looking into losses caused by weeds. The trials consisted of four weed-control treatments which were tested in Forbes over the three years. These were:

  • Nil spray – no summer spray, only a knockdown applied before sowing
  • Miss first spray – when the first initial spray for the fallow period is not applied
  • Full spray – a ‘zero tolerance to weeds’ approach, with herbicide applied about 10 days after a significant rainfall event (20mm or more)
  • Delayed spray – when herbicides are applied about 24days after a significant rainfall event.

It found that every dollar spent controlling summer weed growth in fallow, returned up to a $7.20/ha benefit with the ‘zero tolerance to weeds’ approach (Berry and Dixon 2014). It also found that for every mm of soil moisture lost via summer weed growth, a further 0.56kg/ha of mineral nitrogen was made unavailable to the following crop. This means that if an extra 75mm of moisture was conserved by controlling summer weeds, then there would be an extra 42kg/ha of available N for the following winter crop. This increase in N is not only because of the reduced nitrogen use of weeds but also due to the increased moisture at the surface, which promotes mineralisation and therefore N production. The trials also found that as a profit driver, summer weed control was found to be more important than stubble management because clean, weed-free fallows increased both moisture and nutrient availability.

Stopping weed growth not only increases the yield through conserving moisture and nitrogen, but it also increase yield through:

  • A wider and more reliable sowing window
  • Reduced levels of weed vectored diseases and nematodes
  • Reduced levels of rust inoculum by interrupting the green bridge needed for spore survival
  • Reduced levels of diseases vectored by aphids that live on weed hosts
  • Reduced physical impacts on crop establishment (Cameron and Storrie, 2014).

Timing of weed control has a great impact on fallow moisture storage - the greater the delay in control, the greater the potential economic loss. With this in mind, summer weeds should be controlled when small and actively growing, as this prevents weeds from draining stored soil moisture and nitrogen when they grow. It also lowers the amount of herbicide needed and generally increases the effect of the herbicide.

If you would like any help with weed control over the coming season please do not hesitate to contact one of our agronomists via or chat with one of our team by phoning (07) 46600400 or visiting us in store.