With current moisture conditions and record high fertilizer prices, growing a pulse crop has some advantages. There are many factors to consider when choosing a field to grow a pulse including: rotation history, residual nitrogen levels, weed pressure/species, herbicide carryover issues and topography/soil type.
Crop rotation is one of the most effective ways to help prevent Aphanomyces and root rot diseases in both peas and lentils. Root rot can infect the plant at any stage and once infected there is no way of stopping the disease. Peas and lentils should only be grown 1 in every 4 years at a minimum. Once Aphanomyces has been confirmed in a field, the crop rotation should be increased to 1 in every 6 years minimum (ideally 8 years).
Residual Nitrogen in Soil
Pulses will fix the majority of the plants’ nitrogen requirements if inoculated properly. With the drought conditions and poor yields in many areas in 2021 there has been fields with very high residual nitrogen levels in the soil tests. Pulses grown on fields with high residual nitrogen have the risk of excessive growth leading to lodging and disease, and poorer nodulation. The concern with poor nodulation is that the pulse crop will take the “free” nitrogen on high residual fields and not produce nodules. The plant may run out of nitrogen later in the season and can not fix the nitrogen they need due to poor nodulation. Research done by Guy Lafond (Agriculture and Agri-Food Canada) has shown that pea fields with up to 54 lbs/ac of residual nitrate did not have any impact on yield.
The next question is whether pulses grown on fields with very high residual nitrogen need to be inoculated? Results from a study done by Bremer et al (1989) showed that even when high rates of nitrogen were applied on lentils (up to 80 lbs/ac) the inoculated treatments were always the highest yielding (Figure 2). Keep in mind that a 50 bu/ac pea crop requires approximately 150 lbs/ac of nitrogen, so inoculation is still important.
Even though fields with high residual nitrate (up to 54 lbs/ac) did not have negative impact on yield, it is still economically beneficial to select a field with lower nitrate levels for a pulse, especially with high fertilizer costs.
Pulse crops are not competitive and do not have the broad-spectrum herbicide options that cereal or canola crops do therefore, selecting a clean field is critical. Fields with perennial weed issues such as Canada Thistle or Perennial Sow Thistle should be avoided as there are no in crop control options for these weeds. Other weeds to avoid are Group 2 resistant weeds such as kochia or cleavers.
Herbicide carryover issues/recropping restrictions
Drought conditions increase the risk of herbicide carryover issues because soil moisture is a major factor in soil residual herbicide breakdown. Flucarbazone and clopyralid are two examples of herbicides that have potential carryover issues in peas. Halauxifen and pyrasulfotole are two examples of herbicides with re-crop restrictions on lentils. Make sure to check the re cropping restrictions on any herbicides that were applied on a field.
Peas and lentils are harvested very close to the soil surface, so avoiding fields that have huge topography changes or lots of rocks is important. Peas and lentils have low salinity and water-logged tolerances. Fields with high salinity should also be avoided.
Many factors play a role when selecting a field to grow a pulse crop on. Try to have a crop rotation minimum of 4 years and if Aphanomyces is a concern increase that to 6-8 years. Fields with nitrate levels around 55 lbs/ac have not shown a yield reduction, but it would be more beneficial to grow a cereal or canola crop on high nitrate fields, especially given the current fertilizer prices. It is important to inoculate pulse crops even under high nitrate levels. Pulse crops should be grown on non-saline fields with low weed pressure. Always check the re cropping restrictions and possible herbicide carryover risks on fields prior to seeding.
Precision Agronomist & Field Scouting Manager