It's no longer a sure thing that growers should control summer-growing annual weeds to conserve moisture.
According to senior field crop consultant with PIRSA Rural Solutions in SA, Graham Fromm, there is evidence that during summer moisture in the top 25 cm of soil will be lost through evaporation irrespective of the presence of annual weeds.
"Unless sufficient rainfall to wet the soil beyond 25 cm is recorded in one event, the amount of moisture stored is likely to be limited," Mr Fromm said.
"For a sandy soil it takes about 12 mm of rainfall in one event to wet soil to 25 cm. For loamy sand, 14 mm is needed, and for a sandy clay loam, 25 mm.
"Therefore, given that moisture to 25 cm is likely to evaporate in summer, controlling annual summer weeds after small rainfall events may not be warranted for reasons of moisture conservation alone.
Mr Fromm said because it cost about $15/ha to control summer-growing weeds with herbicides, potential benefits needed to be clearly identified.
Other reasons for controlling summer weeds
There still may be good reasons to control summer annual weeds. These include avoiding machinery blockages, minimising seed contamination of wool and reducing weed seed banks, nutrient losses, and root disease carryover. Some weeds are toxic and should be controlled for that reason. One such weed is heliotrope. An increasing number of grain buyers are reviewing their receival standards, some having a nil tolerance policy for heliotrope.
Different story for perennial weeds
"In the case of perennial weeds such as skeleton weed and silver leaf nightshade, control of above-ground growth during summer months has given significant response in the following crop," said Mr Fromm.
"The reason for this is not clear but there is some evidence that the size of the response to the control of perennial weeds over summer may depend on whether growing-season rainfall is sufficient for the needs of the crop."
Graham Fromm is coordinating a two-State research initiative aimed at finding better and more cost-effective ways of controlling summer weeds.
Weeds such as heliotrope, melons, wireweed and skeleton weed are causing significant management and economic losses for graingrowers in Victoria and SA. Research results are also likely to benefit growers in southern NSW.
Summer weed control has been identified as a high priority by the southern panel of GRDC and grower linkage groups, with growers, through the GRDC, investing about $600,000 in the program over four years.
Mr Fromm said the research aimed to produce:
- cost-effective management practices
- an accurate determination of the benefits flowing from timely control of summer-growing weeds, and
- an improved understanding of the biology and ecology of targeted summer-growing weeds.
Contact: Mr Graham Fromm 08 8535 6400