14 March 1996

Plan a weed-free start to beet

By Robert Harris

SUGAR beet growers should aim for as near a complete weed kill as possible when planning their herbicide programmes. Too many stick to a rigid budget yet leave enough weeds to affect yield.

"The first philosophy should be getting the weed control to get the yield," Morley Research Centres Mike May told a BASF technical briefing last week.

Just one tall weed/sq m can reduce yields by up to 11%, according to Mr May. That is worth £280/ha in a 55-60t/ha crop, making herbicides a very effective input, reckoned Martin Lainsbury, BASFs technical services manager.

Pre-emergence herbicides are important, even in dry years like 1996 said Mr May. "Perhaps they werent as effective as in most years, but they werent a complete failure. Consider them as part of a programme – pre-ems add flexibility to later spray timings."

Many growers who omitted them ran into trouble, he said. Late frosts delayed post-emergence applications. "Where no pre-em herbicide was used, weeds were getting to be a problem."

Post-emergence sprays

The efficacy of post-emergence sprays can be dramatically improved by taking account of recent weather patterns, said Mr May. Previous rainfall determines crop and weed growth. "If it is dry, you get a wax build-up, and a reduced post-em effect." Growers should adjust mixes accordingly, perhaps by adding oil.

A warm rainy period promotes fast, less waxy growth. "In essence, you get good weed control. But when things are growing fast, you dont need oils. If you put them in, you are going to damage the crop. Look at what beet is doing rather than weeds. If the crop is growing rapidly, so are the weeds."

Accurate forecasting also helps. Relative humidity is the key to success – in trials during hot, settled weather, spraying in the early morning gave a near 100% weed kill, making it an ideal tactic for targeting fields with hard-to-kill weeds, or to use cheaper chemistry to peg costs, he explained.

After a rapid fall in humidity during the morning, control fell to 20% after midday in some cases, and only partly recovered in the evening. Growers should either stop spraying, or adjust the mix where possible to increase efficacy.

Hit weeds pre-emergence and take full account of the weather, say specialists. Suffolk grower Ron Gabain agrees. Good weed control in his 173ha of beet last year cost £116/ha and led to yields of up to 60t/ha.


WHY PRE-EMERGENCE?


&#8226 Sensitises weeds to post-emergence sprays – increases efficacy, could use mixes kinder to the crop.

&#8226 Kills first weed flush.

&#8226 Concentrates next weed flush.

&#8226 Post-em timing less critical.

&#8226 Can cut number of sprays.

&#8226 Can kill weeds tricky to tackle with post-emergence sprays.