Wheat yield doubled at Parkes, NSW in 2010 after full summer weed control (top) vs. no spray (bottom)
Recent rains across south-east Australia will provide valuable soil moisture for the 2015 season, but only if summer weeds are kept in check.
GRDC-funded research has shown up to 50 to 80 per cent of yield potential at harvest can be attributed to summer rainfall and summer weed control and its effect on soil moisture and nitrogen availability.
New South Wales Department of Primary Industries research and development agronomist Colin McMaster says even in a wet season, where soil moisture is less critical, the difference in nitrogen from controlling summer weeds will still significantly affect yields.
“The 2010 year was a very wet season, so we expected to see less effect of summer weed control on the crop yields in trials. However, where summer weeds were well controlled, we saw a double in yield potential, from two to four tonnes per hectare, which was caused by the additional nitrogen that was available to the crop,” he said.
Not only does the increased N and soil moisture directly help crop performance, but there are interactions between the two that provide further benefit – increased nitrogen availability improves water use efficiency, and higher soil moisture improves N mineralisation.
“This all means that in a dry year, there’s a huge benefit from the increased soil moisture and N, and in a wet year, the increased N still provides a strong benefit. Controlling summer weeds has a high probability of improving yields regardless of what type of season 2015 turns out to be,” Mr McMaster said.
Control after rain
While the effect of recent rainfall will vary depending on soil type and location, most of the southern region received sufficient rain in January for weeds to germinate.
“It’s important to remember that weeds are easier to kill when they’re small, they require lower chemical rates. Some growers might be thinking they’ll wait a bit longer to ensure all the weeds have already germinated,” Mr McMaster said.
“The problem might be if there’s another rain, those emerged weeds will grow rapidly to a point where they are more difficult to control, or else, the soil will dry out, the weeds will be stressed, and again they will be more difficult to control.”
There are different rules of thumb for different regions for how long growers should wait to spray after a summer rain. In central-west NSW, where Mr McMaster is based, he recommends waiting approximately 10 days before starting spraying. This allows growers time to logistically get over their property in a timely fashion when most weed seeds have germinated, but not too long a delay for the plants to become difficult to control.
“In a lot of regions, the January rainfall will really help store moisture deep within the soil profile, safely below the evaporation zone. As we get closer to sowing it’s important to continue controlling weeds to further increase moisture and nutrient availability, as well as improve moisture conditions for timely sowing.
“Even if the ideal time to spray has passed, late spraying will still prevent nitrogen and moisture from being tied up in the plant and put growers in a better position when it comes to seeding.”
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