Early harvest-time spraying builds moisture bank
GroundCover™ Issue: 107 | Author: Melissa Williams
A harvest-time knockdown to kill summer weeds has been shown to conserve significant soil moisture prior to seeding in a series of trials across the eastern wheatbelt this year.
This moisture conservation response in the root zone (to a depth of 40 centimetres) of sprayed plots ranged from about nine millimetres at Doodlakine up to about 60mm at Southern Cross (see Table 1).
Spraying-out summer weeds concurrently with the harvest also improved early crop vigour.
The nine summer spraying trial sites were set up on a range of soil types across the Kwinana East port zone as an initiative of the local GRDC Regional Cropping Solutions Networks (RCSN) group.
Trial coordinators Precision Agronomics Australia (PAA) enforced a complete summer weed kill using glyphosate.
In January and February this year, moisture testing was carried out on soil core samples to a depth of 80cm by the Department of Agriculture and Food, Western Australia (DAFWA) in Merredin.
CSBP tested these samples for nitrate, ammonium, pH, EC and aluminium. Electromagnetic data was also collected that showed clear differences between sprayed and unsprayed plots.
DAFWA researcher Caroline Peek is preparing to harvest the trials and will compare crop yields from sprayed versus unsprayed summer weed plots at each location.
For Merredin grain grower Mick Caughey, the trial on his property’s calcareous loam had an extra 25mm of soil moisture to a depth of 40cm in February where weeds were controlled in December, compared to the unsprayed area.
He says wheat head counts in September indicated crop yields on plots that were sprayed for summer weeds would be double the yields from unsprayed plots.
“The trial has validated our practice of controlling summer weeds to conserve moisture, even though this can be a costly exercise in light of often tight cash flows,” he says.
“In the eastern wheatbelt, soil moisture is our most precious asset and we need to prioritise its value.
“The trials are indicating that the costs of spraying summer weeds at harvest will be more than recovered the following season.”
Mick and his wife Kate employ a contractor at harvest time to undertake a single spray for dominant summer weeds – lovegrass, paddy melons and caltrop – while they are small.
On the back of 100mm of rain in November and 35mm in December 2012, they used a brew of 1.4 litres/ha glyphosate, 500mL/ha low volume 2,4-D esters, metsulfuron and triclopyr to combat a high summer weed germination. Low volatile esters can be used in WA in summer, but check product label rates.
Mick says if rain arrives again at harvest this year, on the back of a wet spring, summer weed burdens in paddocks will be the driving force behind which cereal crop paddocks are harvested first.
“Seeing in the RCSN trials how much moisture we can retain by controlling summer weeds early, we will be making sure we spray them out as fast as we can this harvest before they suck out too much water,” he says.
PAA consultant Aidan Sinnott says economic analysis of the eastern wheatbelt summer spraying trial data after harvest will shed more light on the cost-benefits of controlling summer weeds.
“A spray at harvest costs about $15/ha and we aim to show how many millimetres of water can be saved per day from when summer weeds start germinating to highlight the returns from using this strategy,” he says.
“We will also be further investigating the most cost-effective time to spray and changes in crop water use efficiency, nitrogen use and disease from sprayed versus unsprayed crops.”
08 9072 0542,
08 9041 1138,
0429 441 067,
RCSN Kwinana West, Kwinana East, Albany and Esperance port zones,
0447 261 607,
A fact sheet on Summer fallow spraying is available at: www.grdc.com.au/GRDC-FS-SummerFallowSpraying
GRDC Project Code SDI00015