Measuring intra-field variability in pea protein to understand influencing factors in Montana cropping systems

In most instances across the individual field sites, field zone (topography or N management) and pod location within the plant influenced final grain yield and grain protein (Tables 1-3). At the Sun River dryland site that was managed under a dryland pea program for 2020 that has historically had variable rate N applications applied during previous winter wheat crops, N management did not affect grain yield but did influence observed grain protein (p = 0.0034). Additionally, the pod location within the plant altered observed grain protein (p = 0.037). The field zones that received the highest historical N rates resulted in a grain N% of 3.48%, while the zones that did not receive N resulted in a 3.30% grain N content for the 2020 field pea crop. Additionally, the bottom pods displayed a grain N% of 0.13% points greater than the top pods, most likely as a result of pod formation beginning with the lowermost pods and working up the plant during the reproductive stages.

At the Sun River irrigated field site, topography (bottom vs top of hill slope) influenced both grain yield (p = 0.0004) and grain N% (p = 0.0251) with pod location only affecting grain N% (p = 0.0001) (Table 2). The top hill zone resulted in a grain yield of 921 kg ha^-1 more than the bottom hill zone. The top hill zone also had a greater grain N% content of 0.18% points over that of the bottom hill zone. Pod location resulted in a larger within-plant grain N% difference than that of the Sun River dryland site as the bottom pods had a 0.33% grain N% content greater than the top pods from the same field.

At the Chester site, which was our single Austrian winter pea field, all explanatory variables influenced grain yield and grain N% content, with a significant interaction between slope aspect and pod location for grain N% content (p = 0.0984) (Tables 3 - 5). Our observations and yield data indicate that a North facing field zone yielded the most grain, followed by a zone with no slope (Flat) (Tables 3 - 4). Additionally, the North facing field zone reduced the top vs. bottom pod grain N% difference previously seen at the Sun River field sites as there was only a 0.02%-point difference between the top and bottom pods sampled in the North facing slope zone (Table 5). Although the results from this field site are intriguing, we believe some of our results are confounded with a possible nodulation failure as many of the plants that were sampled during our in-person scouting trip did not have any viable N-fixing root nodules. However, due to extreme heat experienced during vegetative stages of pea across Montana, early abortion of nodules might just have been unavoidable during the 2020 growing season, regardless of the best management practices followed.

In terms of grain yield vs. protein relationships across our field sites of interest, we observed that there is not a consistent relationship among MT growing regions, although varying management (dryland vs. irrigated) resulted in little to no relationship change (Figure 4). Taking into account the three field sites that grew yellow spring pea, historically lower yield potential growing regions in MT (such as Sun River sites in this study) may have a future advantage over higher precipitation regions due to a positive yield vs. protein relationship that shows there is the possibility to grow a high yielding pea that simultaneously yields a relatively high grain protein content (Figure 4), especially if pea buyers continue to implement premiums for delivery of high protein peas.

Finally, we found that out of the three yellow pea field sites the Sun River irrigated field site had a protein variability difference between the field and plant-scale samples (p = 0.0943) (Figure 5). This tells us that management practices such as increased seeding rates should be evaluated to reduce plant height and in turn, reduce the number of pods per plant to reduce observed protein differences between top and bottom pods. By reducing the within-plant protein variability, the observed field-scale variability is reduced, leading to more consistent deliveries of pea grain to a buyer or fractionation facility that results in the producer receiving a protein premium, in some cases up to USD $1.00 over market price, or at the very least, avoiding rejection due to low protein pea grain.

Tables and Figures

Table 1. ANOVA and Least Square Means with Tukey HSD letter differences for Sun River dryland field site from 2020 sampling.

Table 2. ANOVA and Least Square Means with Tukey HSD letter differences for Sun River irrigated field site from 2020 sampling.

Table 3. ANOVA results for Chester Austrian winter pea site from 2020 sampling.

*CLD letters denote a significant difference at alpha = 0.10 via Tukeys HSD. 

Table 4. Least Square Means and Tukey HSD letter differences from Chester Austrian winter pea site from 2020 sampling for grain yield.

*CLD letters denote a significant difference at alpha = 0.10 via Tukeys HSD. 

Table 5. Least Square Means and Tukey HSD letter differences from Chester Austrian winter pea site from 2020 sampling for grain N% content.

*CLD letters denote a significant difference at alpha = 0.10 via Tukeys HSD. 

Figure 4. Overall grain yield vs. grain protein relationships across three 2020 MT field sites with Pearson’s Correlation results and significance.  

Figure 5. Comparison of grain protein content variability between the field and plant scale for the field zone of interest (top vs. the bottom of hill slope) and pod location within a plant.