Pea crops do not compete well against weeds, so good weed management is essential. An integrated approach to weed control combines preventative and cultural measures (such as the use of clean, healthy seed, crop rotation, and seeding rates) with the effective use of herbicides. Weed management for pea crops should be considered in the year prior to growing pea. Grow a rotational crop that provides good competition to weeds, allows for a wide range of herbicide options, and is easy to control as a volunteer in the preceding pea crop. Managing perennial weeds, knowing your field’s weed history and anticipating in-crop weeds are keys to a successful weed control program.
An application of pre-harvest glyphosate in the previous year’s crop, fall herbicide application for winter annuals and selection of a field that has weed problems can be controlled culturally or with herbicides registered for use in pea, are important points to note when planning pea production. Avoid areas where perennial weeds such as Canada thistle or sow thistle may be a problem. Pea is susceptible to the soil residues of some herbicides used in previous years.
A spring glyphosate application, either pre-seed or pre-emergent, is recommended. This provides early season weed control and may provide control of weeds for which no in-crop control is available. Pea is slower to emerge, so a wider window between seeding and emergence allows more time if a post-seed/pre-emergent glyphosate application is planned. However, with good growing conditions and shallower seeding, emergence can be quicker than expected, so timing must be watched closely. Seedlings can be damaged as early as soil cracking as they are emerging and come into contact with glyphosate.
More recent herbicides registered for pre-seed or pre-emergent weed control in pea include CleanStart® and Heat®. They can offer enhanced weed control of certain weeds (eg: glyphosate tolerant canola, wild buckwheat) and may result in faster weed burndown.
Herbicide resistant weeds: Herbicide choice should take into account herbicide rotation to slow the development of resistant weeds. Resistant weeds can have a huge impact in pea crops simply because it is a less competitive crop and there are limited herbicide options. A few examples of herbicide resistant weeds that are particularly troublesome for pea growers include Group 2 resistant kochia, wild mustard and cleavers, as well as Group 1 and Group 2 resistant wild oat. Rotating herbicide groups away from Group 1 and 2 products, especially in rotational years where pea is not grown, can help prevent or manage resistant weeds. BASF recommends that Group 2 products be applied no more than twice in a four year period, and never twice in the same year. Research indicates that alternating between two modes of action for wild oat control will double the number of years for resistance build-up, and alternating with a third mode of action will increase the time of resistance build-up to four times as long as for a single mode of action for wild oat control.
Timing of application: Timing of herbicide application is very important with peas. Earlier herbicide application means weeds are well-exposed, are smaller (generally weeds are easier to control at a younger stage) and the crop is less susceptible to injury. Research in northeast Alberta and the Peace River region showed pea yields were higher and more consistent with spraying at the second node stage compared to the eighth node stage. Yield was increased (22 to 125 per cent) seven times out of 10 with early applications. Research conducted at Agriculture and Agri-Food Canada and Agri-Arm Sites in Saskatchewan also demonstrated the importance of early weed removal in pea production. Annual weeds were removed from pea crops at intervals of one, two, three and four weeks after crop emergence. Pea yields declined by 0 per cent after one week, seven per cent after two weeks, 12 per cent after three weeks, and 26 per cent after four weeks.
Minimizing crop injury to herbicides: During periods of crop stress (heat, drought, frost or after land rolling) the ability of the pea crop to tolerate herbicide application may be reduced. Crop injury can be reduced by waiting approximately four days after the crop stress occurs before applying herbicide, by maintaining water volumes at label recommendations, and by applying the product during the evening.
Pea can be damaged easily by some herbicides registered for other crops. Sprayer tanks should be thoroughly cleaned before applying any crop protection product to pea and care taken not to drift herbicides from other fields onto pea fields.
Alternatives to chemical weed control: Non-herbicide options may be considered as well. Tillage can have a beneficial effect for control of some weeds while having the opposite effect on others. Tillage may be a tool to reduce kochia populations. Kochia appears well adapted to no-till with germination beginning at 50 cumulative growing degree days (well before other common weed species). Burial of kochia seed to at least one cm or deeper can result in reduced germination or death of the germinated seed prior to emergence. Tillage to bury kochia seed should not be overlooked as a part of an integrated weed strategy for kochia control. However, this has limited value where minimum or no-till is practiced.
Rod-weeding five to seven days after seeding provides excellent weed control without herbicide use and good tolerance to peas seeded 7.5 cm (3in) deep. Tillage 10-12 days prior to seeding helps stimulate weed growth for control with the rod-weeder.
Harrowing between seeding and emergence of the pea crop can control newly emerged weed seedlings and remove weeds that escaped previous tillage operations. Harrowing should be avoided during crop emergence and for several days afterwards to permit effective rooting and stand establishment. Post-emergence harrowing has been researched at the Agriculture and Agri-Food Canada Research Station at Scott, Saskatchewan. Pea was found to be somewhat tolerant to post emergence harrowing. A higher seeding rate should be used to offset the plant losses due to harrowing. Post-emergent harrowing should be done under warm, dry conditions to improve weed control and to prevent the spread of diseases.