By Donna Fleury
Field pea is regularly grown in rotation with wheat and other cereals in Western Canada. Generally, the recommendation is for three consecutive years between pulse crops to minimize plant disease risks. However, there are many agronomic and economic benefits of including grain legumes such as field pea in rotation, and growers expressed interest in the mid-1990s in increasing the frequency of growing pulses.
Researchers at the Agriculture and Agri-Food Canada (AAFC) Indian Head Research Farm in Saskatchewan conducted a 10-year study to evaluate the risks of intensifying rotations. “When we started the study in 1998, we were trying to answer questions for farmers about the risks of increasing field pea frequency in rotation,” explains Dr. Guy Lafond, scientist, Production System Agronomy. “Growers may rent or purchase land that had recently been seeded to pulses, or consolidating fields and expanding acreage, or simply wanted to grow field pea more often to take advantage of economic opportunities. We wanted to determine what the risks and impacts would be on agronomic performance.”
The research, conducted at the Indian Head Research Farm from 1998 to 2007, evaluated three crop rotations with field pea (FP) and wheat (W) over the 10-year period, including continuous pea (C-Pea), W-FP and W-W-FP. The objectives of this study were to examine the effects of increasing the frequency of field pea in cropping systems with wheat on crop establishment, grain yield, crop water use, and grain quality of both field pea and wheat.
The plots were seeded as early as possible each year and on the same day. For field pea, a seeding rate of approximately 190 kg/ha was used and a granular, peat-based Rhizobium leguminosarum inoculant was applied with the seed at a rate of 5.6 kg/ha. No fungicidal seed treatments or foliar fungicides were used in either the wheat or field pea phases of the rotation.
In the field pea phase of continuous pea and wheat-field pea rotation, starter fertilizer N treatments consisting of 5, 20, and 40 kg/ha N were added as urea. The field pea plots present in wheat-wheat-field pea received no starter N. However, mono-ammonium phosphate, which adds approximately 5 kg/ha N per year, was applied at a rate of 10.3 kg/ha P in wheat and field pea annually. The wheat plots received 80 kg/ha N as urea every year. All N (when used) and P fertilizers were side-banded at time of seeding (2.5 cm to the side and 7.5 cm below the seed) for both wheat and field pea.
“Overall, the research shows that there is very little risk to increasing the frequency of field pea in rotation in the short term,” explains Lafond. “This provides flexibility for growers to shorten rotations when the price is good, for example, and they want to take advantage of marketing opportunities, or changes in their cropping management.
This strategy offers flexibility over the short term, but is not something that should be done continuously.”
Weeds were one of the biggest agronomic problems of increasing the frequency of field pea over the long term. “Over the years of the study we observed that weed control was always more challenging as the intensity of FP in the cropping system increased,” says Lafond. “Therefore, we recommend that the intensification should be more short term in nature to avoid difficult weed control situations. Diseases can also become more of a problem as the frequency is increased.”
The results showed that the intensification of field pea in a wheat cropping system did not result in adverse effects on plant densities in field pea after 13 years of continuous pea. The overall observed plant densities for field pea averaged 65 plants per square metre, which falls into the recommended range of 50 to 75 plants per square metre.
Plant densities for W averaged 307 plants per square metre, which exceeded the recommended range of 200 to 250 plants, and did not reduce yields, even in dry years. Equivalent field pea plant populations were observed among the other rotations. The plant densities were obtained without the used of a fungicidal seed treatment.
Crop rotation and starter N generally had similar effects for field pea and wheat plant densities. The similar field pea plant densities regardless of the frequency of field pea in the cropping system would suggest that adjustments in seeding rates are not required. Some starter-N benefits were observed in the wheat-pea sequence but not the continuous pea sequence. Grain protein was higher in the continuous pea than the average of the wheat-pea and wheat-wheat-field pea, likely the result of lower overall grain yields. Higher grain protein in field pea was observed with the starter N rate of 40 kg/ha N in both continuous and wheat-field pea rotations, but not at the N rate of 20 kg/ha. Grain protein concentration was greater for wheat on field pea than for wheat stubble.
“Overall, wheat grown on field pea stubble yielded about four percent more than wheat on wheat stubble,” says Lafond. “The yield difference between continuous pea and wheat-field pea was the same regardless of whether growing season precipitation was below or above average. When we compared the yields of field pea in wheat-field pea and wheat-wheat-field pea with the yields of wheat in the same rotations, the average yields were the same for field pea (2365 kg/ha) and wheat (2307 kg/ha).”
The impact on agronomic performance of shortening rotations also depends on location and climate conditions. “In the drier areas in Saskatchewan, some growers follow a durum-lentil rotation and have been doing that for a long time,” says Lafond. “The climate is a bit drier in those areas, so that helps reduce the disease risk. However, over the long term, weeds could become a problem. In the wetter areas of the province, diseases could more quickly become a problem with too intensive of rotations.”
“Based on the results of this study, intensification of field pea in cropping systems is feasible using a cereal-pea rotation providing for a one-year break between successive field pea crops,” says Lafond. “However, from a crop management perspective, the recommendation would be to intensify only intermittently to take advantage of marketing and agronomic opportunities such as field consolidation. Growers have the flexibility to adjust their rotations to take advantage of the agronomic and economic benefits of growing field pea, as well as contributing to a reduction in greenhouse gas emissions.”