WITH unpredictable weather, correct use of the reliable and effective oilseed rape desiccant, glyphosate, will be a key to success this season, Roundup technical specialist, Barrie Hunt, told The SF.

He said farmers should be prepared for another year of snatched spraying opportunities and challenging summer weather, pointing out that the optimum desiccation timing for modern hybrids is significantly later than for traditional, denser pure line stands.

Mr Hunt insisted that growers should not be tempted to spray too early in an attempt to hurry the harvest with hybrids, even though the thick-stemmed crops of today are a lot more demanding to desiccate than those of the past.

“A thicker 23mm diameter stem, for instance, has nearly two and a half times the volume of a traditional 15mm stem,” Barrie Hunt noted.

“There’s 50% more internal plant tissue to deal with for every extra square centimetre of surface area, placing particular demands on the glyphosate regime.

“Holding off with the sprayer will not delay combining and could make all the difference to maximising yields and oil contents,” he added.

“What’s more, today’s pod shatter resistant varieties mean this can be done without increasing the risk of seed losses in the run up to and at harvest.

“Our work clearly shows that earlier desiccation does not mean earlier combining.

“Desiccating too early just means stems take longer to dry down – especially if they’re thick.

“It may also noticeably restrict output – primarily by limiting the oil which is formed relatively late in seed-fill – as well as increasing the risk of red seed at harvest.”

Monsanto studies also show that the greater branching of modern hybrids grown at recommended plant populations means a much higher proportion of the yield comes from side branches which mature significantly later than the main raceme.

Crops with average populations of 30-40 plants/m2 were typically found carry 80% or more of their yield in side branches and seeds with a 5% higher moisture content than those on the main raceme.

This means spray timing must be based on assessments of pods from the area of the crop where the bulk of the yield is being carried, not the main raceme.

In practice, timing can only be determined by taking a representative sample of 20 pods and checking for the tell-tale change of seed colour from green to brown using the guidelines developed by Monsanto.

“Of course, it’s also vital to use the correct dose rate and appreciate that the addition of an adjuvant cannot replace insufficient glyphosate for the job in hand,” Mr Hunt added.

He recommended:

Use water volumes of 200-250 litres/ha in thick or leaning crops;

Spray early in the day in hot weather to take advantage of higher relative humidity;

Adjust booms to ensure the best coverage of the whole crop;

Employ low drift nozzles wherever possible.