Lentils are a very poor competitor with weeds. Good weed control in lentils requires a long-term strategy involving the entire crop rotation. Perennial weeds, such as Canada thistle and sow thistle, should be controlled in the years prior to lentil production. Volunteer wheat and barley are difficult to clean from small-seeded lentil and should be controlled in-crop. Weeds that germinate late in the season, such as Russian thistle, kochia, and wild tomato, cause severe competition to the crop. Late emerging weeds interfere with harvesting, increase dockage, and increase staining and moisture levels in the harvested seed.
A late fall application of a phenoxy herbicide such as 2,4-D or MCPA can be used at rates of 280 grams active ingredient per hectare (g ai/ha) or less to control winter annual broadleaf weeds in fields planned for lentil production. Spring applications and/or applications that include dicamba should be avoided to prevent possible crop injury.
Pre-seed or pre-emergence burnoff are important for controlling weeds before the crop emerges. Soil applied products may provide early weed control during early crop growth but usually require moisture for activation. For in-crop applications of herbicides the critical weed free period for lentils is between two to five nodes of development.
The introduction of Clearfield® technology has given lentil growers additional in-crop herbicide choices. Research on Clearfield® lentils concluded the optimum timing for herbicides was between the five to six node stage and the 10 node stage. Weeds emerging after the 10 node stage were less likely to reduce yield but may lead to harvesting issues.
Make sure to follow label directions and apply the herbicide at the correct time of plant development. Some herbicides can move in the soil after heavy rainfall, so if the use of these products is anticipated, lentils must be planted at least 5 cm (2 in.) deep to prevent injury to seedlings. For example, do not use Sencor® in soils with less than four per cent organic matter.
EdgeTM and trifluralin (Rival®, TreflanTM) are registered only for fall application. A spring application, especially in lighter soils, can cause seedling injury and may lead to increased incidence of seedling diseases such as rhizoctonia wire stem.
Herbicide-resistant weeds: Herbicide rotation is an important step in slowing the development of resistant weeds. Resistant weeds can have a significant impact in lentil crops simply because lentils are a less competitive crop and there are limited herbicide options. A few examples of herbicide resistant weeds that are particularly troublesome for lentil 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 years where lentil is not grown, may help manage resistant weeds. 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.
Minimizing crop injury to herbicides: During periods of crop stress (heat, drought, frost, or after land rolling) the ability of the lentil 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.
Lentils can be damaged easily by some herbicides registered for other crops. Sprayer tanks should be thoroughly cleaned before applying any crop protection product to lentils and care taken to minimize drift of herbicides from other fields onto lentil fields.
Alternatives to chemical weed control: Non-herbicide options include tillage, harrowing, and higher seeding rates. Tillage may be a tool to reduce kochia populations. 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.
Alternatives to chemical weed control research completed at Agriculture and Agri-Food Canada, Scott showed that post-emergent harrowing with a tine harrow can be used to control weed seedlings when the crop is very short (less than 10 cm), provided that the foliage is dry and the operation is done on a warm, sunny day. An increased lentil seeding rate should be used to offset the plant losses during harrowing.
Higher seeding rates are a great option for improving weed competitiveness. Research by Dr. Steve Shirtliffe at the University of Saskatchewan suggests higher seeding rates up to 240 plants per square metre reduced weed populations and increased lentil yields.