Response to Seeding Rate and Insecticide Seed Treatment

Objective

The overall intent of the Alberta Pulse Growers – Plot2Field (P2F) initiative is to transfer small plot research to a field scale basis and assess the quality of data obtained. The P2F research described in this article tests the hypothesis that the current recommended field pea seeding rate of 7-8 plants/ft2, based on a conventional system in small plot research, will still provide the highest yield in minimum/no-till systems on a field scale basis.

Trial Set Up and Data Analysis

Three seeding rates were tested in a randomized complete block design with three replicates: a low seeding rate of 4-5 plants/ft2, the recommended seeding rate of 7-8 plants/m2 and a high seeding rate of 9-11 plants/ft2. CDC Meadow, yellow pea was used at all sites with an insecticide seed treatment applied to help manage pea leaf weevil. The seeding rate trial was conducted in 2017 and 2018 at multiple sites across Alberta. There were 18 site-years of data collected and all trial cooperators used a seed drill with a 10” row spacing. However, three site-years were removed from the overall analysis due to seeding rate treatments being incorrectly achieved. This resulted in 15 site-years of useable data for the seeding rate study. The insecticide seed treatment component was added to the study in 2018. The comparison was between seed treated with an insecticide to help manage pea leaf weevil and untreated seed. This comparison was made at the recommended seeding rate of 7-8 plants/ft2. The additional treatment was implemented at eight site-years. However, three site-years were removed from the overall analysis due to seeding rates being incorrectly achieved. This resulted in 5 site-years of useable data for the insecticide seed treatment study. Multi-year and individual site year analysis was conducted using SAS 9.4, SAS Institute Inc., Cary, NC. For all measured variables, the analysis of variance (ANOVA) indicated there were significant differences between sites. The ANOVA also tested for significant seeding rate or seed treatment effects and site-year x treatment interactions. The LSmeans are reported. Where there were significant treatment differences, a Tukey mean separation was used to determine differences between treatments.

Yield Response

The average yield was 58.5 bu/acre; however, yields ranged from a low of 33.9 bu/ac at Willow Creek in 2017 to a high of 93 bu/ac at Kneehill County in 2017. There was no significant yield response to the increasing seeding rates found in the multi-year analysis (Figure 1) or at individual site years. Meaning all seeding rates were equally good. Although there are economic savings with lower seeding rates, there are other agronomic benefits associated with the higher seeding rates. There was also no significant yield response to the insecticide seed treatment compared with no seed treatment (Figure 1). The 2018 pea leaf weevil (PLW) surveys found very low levels of PLW damage, with the majority of sites having no visible feeding damage or less than four feeding notches. This explains the lack of response to the insecticide seed treatment.

Pods per Plant Response

The average number of pods per plant was 7.6. The most pods per plant were found at the Parkland County site in 2017 with 13.1 pods/plant. The fewest pods per plant were found at the second Vermilion 2018 site with 4.6 pods/plant. Based on the multi-year analysis, there was a significant decrease in the number of pods per plant as seeding rate increased (Figure 2). This reflected the plasticity of the plants to compensate for increasing plant number. For example, when the target seeding rate was 4-5 plants/ft2, the plants had 9.5 pods/plant. When the target seeding rate was 9-11 plants/ft2, the plants had 6 pods/plant. Analysis of individual site years found this trend at 14 of 15 site-year, but it was not always significant due to the high variability associated with this type of data collection. There was also no significant difference in the number of pods per plant for the insecticide seed treatment compared with no seed treatment (Figure 2). Since plant stands and yield were not impacted by the seed treatment, we expected this lack of difference in number of pods per plant.

Days to Maturity Response

Averaged over all sites, the field peas took 96 days to mature. The shortest maturity was recorded at the 2018 MacKenzie County site with 79 days and the longest maturity was recorded at the Mountain View site in 2018 with 113 days. Based on the multi-year analysis, there was a highly significant decrease in the days to maturity as seeding rate increased (Figure 3). As seeding rate increased, there were fewer pods per plant and these pods had more even maturity. The 9-11 plants/ft2 seeding rate reduced days to maturity by 4.3 days compared with the 4-5 plants/ft2 seeding rate. The insecticide seed treatment compared had no impact days to maturity (Figure 3).

Other Measured Variables

Response to increasing seeding rates

As expected, the plant stands increased in response to increasing seeding rates; however, the plant stands achieved were on the low end of the target (Table 1). Like the trends observed with increasing seeding rates decreasing days to maturity, there was also a reduction in days to flowering associated with increasing seeding rates. Nodule assessments, lodging and protein showed no response to increasing seeding rates.

Response to insecticide seed treatment

Like the previously discussed variables, there was no significant difference in plant stand, nodule rating, days to flower, lodging, or protein between the treated and untreated seed (Table 1). This is attributed to the lack or limited PLW pressure found in the study years.

Table 1. Multi-year analysis of plant stands, nodule ratings, days to flowering, lodging and protein response to seeding rate treatments and insecticide seed treatments.

Multi-Year Analysis	Actual Plants	Nodule Rating (1-13)	Days to Flower (from seeding)	Lodging (1-9)	Protein (%)
Seeding Rates
Number of Sites	15	15	15	15	12
4-5 plants/ft²	4.4 a	11.5 a	55.6 a	4.6 a	19.9 a
7-8 plants/ft²	6.8 b	11.5 a	53.9 b	4.6 a	20.0 a
9-11 plants/ft²	9.1 c	11.7 a	52.1 c	4.6 a	20.0 a
Average	6.8	11.6	53.9	4.6	19.9
CV%	33.6	10.8	7.8	79.5	6.2
Seeding Rate p-value	<0.0001 Significant	0.3607 NS^†	<0.0001 Significant	0.8412 NS	0.8752 NS
Insecticide Seed Treatment
Number of Sites	5	5	5	5	5
7-8 plants/ft² Treated	6.3 A	12.3 A	51.3 A	1.2 A	20.2 A
7-8 plants/ft² Untreated	6.2 A	12.6 A	51.5 A	1.2 A	20.1 A
Average	6.3	12.4	51.4	1.2	20.1
CV%	17.7	8.9	6.8	33.9	6.1
Seed Treatment p-value	0.1442 NS	0.0763 NS	0.8632 NS	n/a NS	0.7039 NS

^†NS = Non-Significant. Values followed by the different lower or uppercase letters are significantly different based on a Tukey mean separation test with a p = 0.05.

Take Home Message

While increasing seeding rates did not increase yield, plants compensated by reducing the number of pods per plant. A plant with fewer pods, matures in a shorter time frame, resulting in a 4.3 day reduction in maturity. The seeding rate treatments did not impact nodules, lodging or protein. While there is an economic cost to increasing seeding rate, there are additional benefits associated with higher seeding rates. These include: increased crop competition against weeds, improved herbicide efficacy, improved uniformity across the landscape leading to increased synchrony for timing crop management activities. If resistance weed management is needed, higher seeding rates can be an agronomic tool for increasing crop competition. Based on the reduction in days to maturity and the crop competition benefits associated with higher seeding rates, it is still recommended to target a pea seeding rate which will achieve 7-8 plants/ft2. There was no benefit in response to the insecticide seed treatment due to the lack or very limited amount of pea leaf weevil presence. It is therefore recommended to only use an insecticide seed treatment where there is anticipated pea leaf weevil pressure.