26 March 1999

Pre-em beet spray now can cut costs later

DONT forget the value of pre-emergence herbicides on sugar beet, says Morley Research Centre.

Even though modern post-emergence products can tackle the complete weed spectrum, a spray now could save yield and herbicide cost later on, says Martin Lainsbury.

"The crucial thing with weed control in sugar beet is timing. Using a pre-emergence product buys some flexibility with post-emergence spray timings."

That could be particularly important on this seasons later drilled crops. Untreated weeds will emerge faster as temperatures rise.

In a range of three-spray herbicide trials at Morley last season, programmes including a pre-emergence dose of chloridazon – for example, Pyramin DF – yielded more. "While the difference between the yields was never statistically significant, it did occur in all trials. And the cost of the pre-emergence programme was marginally cheaper."

But that might not be the case if more expensive products such as Spectron/Magnum (chloridazon + ethofumesate) or Goltix (metamitron) are used, he notes.

Timings not disrupted

The Morley spray timings were not disrupted by the weather, but in many commercial crops last year T1 "cotyledon to 2 true leaf" sprays were seriously delayed due to the wet April. Where pre-emergence sprays were omitted subsequent post-emergence control was poor and more costly.

"Where pre-emergence products had been used they seemed to do their job before the rain washed them away," says Mr Lainsbury.

Subsequent weed emergence is delayed and when they do emerge reduced root mass means they are more sensitive to later post-emergence applications. That allows a wait and see approach with the first post-emergence product, he says.

"Without a residual growers must go at cotyledon stage of the weeds to be sure of control."

On slow to dry out heavy soils that can be impossible in a wet spring. Only on an organic soil with volunteer potatoes to control might the pre-emergence be left out. Then the grower is likely to be making three post-emergence passes to tackle the potatoes anyway. Furthermore much of the residuals activity is lost due to lock-up.

"In general we always advocate using a pre-emergence residual," he concludes. &#42