Chemical Management Of Weeds

Chemical management of weeds includes the use of herbicides and soil fumigants. In the past, fumigants have been used on land where intensive brassicas are grown. In particular, this involved the use of methyl-bromide to destroy soil-borne pathogens and weed seeds. Use of this chemical is being discontinued internationally to prevent further damage to the atmospheric ozone layer, and safer alternatives are being sought.

Herbicides may be selective or non-selective in their mode of action. Selective herbicides destroy specific plant species and may thus be used to eliminate them from populations of other species. Probably the most widely used selective herbicide for Brassica crops is trifluralin (a,a,a-trifluoro-2, 6-dinitro-N,N-dipropyl-p-toluidine). Non-selective herbicides such as glyphosate (N-(phosphonomethyl) glycine) and paraquat (1,1'-dimethyl-4,4'-bipyridinium) will destroy all green tissues that they come into contact with. Thus they are used, for example, to destroy weeds that germinate in stale seedbeds prior to drilling or transplanting the cash crop. These two chemicals are inactivated once they come into contact with soil.

Herbicides can also be classified according to the timing of their application in relation to crop growth stage. These are broadly the pre-emergence and post-emergence categories. Those with pre-emergence characteristics are applied prior to or after sowing the seeds of Brassica crops and prior to their emergence. Post-emergence herbicides are applied once the crop has emerged, to destroy competing weed species.

Well-established pre-emergence herbicides for the control of annual weeds in drilled and transplanted Brassica crops in the UK include trifluralin (a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) and propachlor (2-chloro-N-isopropylacetanilide). Neither kills all the weed species commonly present in these crops so their spectra of activity are largely complementary and they can be applied in combination (Roberts and Bond, 1975). Initially this involved two operations because trifluralin needs to be incorporated into the soil before drilling or transplanting, while it is recommended that propachlor is applied to the soil surface after drilling. In attempts to avoid this level of complexity in crop management, experiments studied the effects of combined applications. Trifluralin can be applied as a granular formulation at between 0.56 and 1.12 kg/ha and propachlor as wettable powder at between 2.18 and 4.37 kg/ha. Applying mixtures and incorporating propachlor into the soil had little effect on the efficacy of propachlor, and cabbage yields were not significantly affected. Incorporation of both chemicals in a combined application using granular formulations appears to be another cost-effective strategy. There may be a need to increase the dose rate of propachlor to compensate for the effects of incorporation. There are very few if any recommended post-emergence selective herbicides that may safely be applied to vegetable Brassica crops. Generally, growers attempt to ensure that crops are established either from seed or by transplanting without weed competition. Subsequently, the crops normally grow speedily enough to close their canopies before weeds become established.

Building Your Own Greenhouse

Building Your Own Greenhouse

You Might Just End Up Spending More Time In Planning Your Greenhouse Than Your Home Don’t Blame Us If Your Wife Gets Mad. Don't Be A Conventional Greenhouse Dreamer! Come Out Of The Mould, Build Your Own And Let Your Greenhouse Give A Better Yield Than Any Other In Town! Discover How You Can Start Your Own Greenhouse With Healthier Plants… Anytime Of The Year!

Get My Free Ebook


Post a comment