Ideally,both combining peas and field beans are direct combined without the need fordesiccation as this adds to costs and application causes crop loss, advisesJim Scrimshaw, PGRO Principal Technical Officer. However, this is not alwayspossible, and some crops which are weedy and/or are ripening unevenly willbenefit from the timely use of an appropriate product to maintain quality.
Whenit comes to choosing a chemical desiccant in peas and beans, the choice isgenerally between glyphosate and diquat, primarily these will be the onesconsidered here. There are, however, glufosinate ammonium products alsoapproved.
Dontforget that desiccants bring the prospective harvest date forward – they dontspeed up seed maturity. The chemical terminates growth when development iscomplete allowing a smoother harvest; and harvest planning and can prevent thesetting of weed seed.
Bothglyphosate and diquat are non-selective materials, Diquat is a true desiccantwhile glyphosate is more a pre-harvest herbicide. A true desiccant rapidlykills above ground growth of both crop and weeds allowing rapid drying andearlier harvest (between 3-7 days after application). It will not givelong-term weed control – particularly of perennials – whereas glyphosate will.
Wherepersistent perennial weeds are a problem, glyphosate is the product of choiceas it is a translocated herbicide acting on plant meristems. Crop effects takelonger to materialise and harvest is brought forward more slowly (10-14 days)compared to diquat.
Glufosinateammonium products are mainly contact acting, but there is some translocationwithin the leaves. Crops are ready to harvest 10 -14 days after application.
Agood coverage of diquat on crop and weeds is essential and it works best whenthe crop is beginning to senesce. Activity is dependent on temperature and UVlight levels while glyphosate needs plants to be actively growing for the besteffect.
Timingof the applications is very important. If desiccation takes place too earlythen effects can be uneven, stems can de-lignify and crops collapse reducingrecoverable yield. Quality may suffer this is more of a problem with peas -possibly increasing undersized grain and greater chances of shelling out due tothe longer delay before combining. Diquat tends to work better on dicot plants(peas and beans) and glyphosate on monocots (cereals).
Applicationsof diquat should be made to pea crops which are turning yellow. The bottom podsshould be parchment like and the seed hard; pods in the middle should bebecoming parchment like, the seed rubbery and squeezable between finger andthumb without splitting; top pods should still be fleshy green /yellow withseed that splits when squeezed.
Ifglyphosate is to be used, a crop moisture content of around 30% is suggestedbefore application.
Desiccationhas a slow effect on the relatively thick green stems of field beans andapplications should be made when 90% of pods are dry and black. It is suggestedthat in beans glyphosate should be used specifically as a pre-harvest treatmentto control perennial weeds.
Andremember that neither glyphosate or glufosinate ammonium products should beused on crops intended for seed,” adds Jim Scrimshaw.