Evaluation of Pulse Crops for Dryland Production

Yield:

Seed yield (kg ha^-1) was compared within each crop. Year was the only significant effect on yield for all crops (Table 1). There were no significant effects from variety, planting, date, seeding rate or any interactions. Chickpea yielded 1669.0 kg ha^-1 in 2018 and 1197.7 kg ha^-1 in 2019 (Table 2). Dry pea yielded 1867.8 kg ha^-1 in 2018 and 1343.7 kg ha^-1 in 2019. Lentil yielded 364.9 kg ha^-1 in 2018 and 1285.8 kg ha^-1 in 2019. 2018 yields were significantly higher in chickpea and dry pea and significantly lower in lentil (Table 2).

Table 1: Results of a general linear model for the determination of significant effects of year, variety, and planting date within each crop type on yield (kg ha^-1). All interactions were analyzed, and no interactions were significant.

Table 2: Effect of year on each crop yield (kg/ha). Means within a column followed by the same letter are not significantly different from each other at the 5% probability level.

Wheat Available Water:

            Available water at wheat planting gives an idea of the impact of the fallow replacement crop on the following wheat crop. Significant effects were seen from depth, crop, treatment (cover crop, grain, fallow), year by crop, year by treatment, crop by treatment, crop by depth, and treatment by depth (Table 3).

Chickpea Wheat Available Water

            Within chickpea, soil depth, year, planting date, and treatment had significant effect on stored soil moisture (Table 4). The interaction between depth and treatment was also significant with cover crop generally having more water than grain except at the 0-10 cm depth in 2019 (Table 5). When looking at all depths, 2019 had a higher amount of water available at all depths (0-10 cm: 223.6 mm, 10-30 cm: 410.6 mm, 30-60 cm: 681.9 mm, 60-90 cm: 719.9 mm) than 2018 (0-10 cm: 120.5 mm, 10-30 cm: 291.3 mm, 30-60 cm: 468.7 mm, 60-90 cm: 647.7 mm). Available water increased in deeper depths. For planting dates, mid planting dates averaged less available water (428.4 mm) than early (457.5 mm) or late (457.6 mm) but were not significantly different than each in all years or treatments.

Dry Pea Wheat Available Water

            Depth, year, treatment, and depth by treatment were significant effects in dry peas (Table 4).  Fallow had the highest average water available compared to both cover crop and grain at 0-10 and 10-30cm depths but was only significantly more than both in 2019 at the 0-10 cm depth. Fallow had the lowest available water in the 30-60 and 60-90 cm depths but was only significantly lower than cover crop in 2019 at 60-90 cm (Table 5). In depths, the 0-10 cm depth averaged the least amount of water (146.1 mm) followed by the 10-30 depth with 299.4 mm (Table 5). 30-60 cm depths averaged 673.8 mm while 60-90 cm available water was lower with 601.6 mm. In 2018, the available average water was higher (440.0 mm) than 2019 (425.3 mm).

Lentil Wheat Available Water

            In lentil, depth, depth by year, and year by treatment had significant effects (Table 4). Average available water increased at deeper depths (Table 5). In 2018, the fallow treatment had the highest available water compared to cover crop and grain while only being significantly higher than both in 2018 at depths of 10-30 and 60-90 cm. In 2019, fallow wheat available water was significantly lower than both grain and cover crop at the depth of 60-90 cm.  In the year by depth interaction, available water was higher in 2019 down to 30 cm when compared to 2018 but lower than 2018 from 30-90 cm. 

Table 3: Results of a general linear model for the determination of significant effects of depth, year, crop, planting date, and treatment type (cover crop or grain crop) on wheat available water (mm). Seeding rate was not analyzed due to lack of significant difference at pulse harvest, so only seeding rate 1 was sampled. All interactions were analyzed, and the significant interactions listed.

Table 4: Results of a general linear model for the determination of significant effects of depth, year, crop, planting date, and treatment type (cover crop or grain crop) on wheat available soil water (mm). Seeding rate and variety were not analyzed due to lack of significant difference at pulse harvest, so only seeding rate 1 and the standard variety (Frontier, Carver, Maxim) were sampled. All interactions were analyzed, and the significant interactions are listed.

Table 5: Results of a least significant difference analysis to separate the means of wheat available water (mm) due to significant effects on wheat available soil water in 2018 and 2019 within each crop. Treatment (cover crop or grain crop) was a significant effect for all crops and planting date was also significant in chickpea (Table 25). There was no cover crop treatment in dry pea in 2018 due to rapid maturity of the crop. Means within a column within a crop type followed by the same letter are not significantly different from each other at the 5% probability level.

Soil Nitrate Change: 

Soil nitrate nitrogen change during the growing season was analyzed to determine the effect of year, crop, planting date, soil depth, and seeding rate on the amount of nitrate used compared to fallow. Soil nitrate was significantly affected by depth, year, crop and the interactions of depth by year, depth by crop, year by crop, and depth by year by crop (Table 6). The data was analyzed by crop type to determine significant effects within each (Table 7).

Chickpea

            Depth, planting date, and treatment type (cover crop or grain crop) significantly affected nitrate change in chickpeas (Table 7). Interactions of depth by year, depth by planting date, and depth by treatment were also significant. For depth, nitrate change levels decreased with deeper depths (Table 8). Early planting dates had a higher average nitrate gain of 2.34 mg kg soil^-1 than mid (0.94 mg kg soil^-1) or late (1.32 mg kg soil^-1). Cover crop treatments averaged the highest nitrate mg kg soil^-1 addition with 4.06 mg kg soil^-1 added on average followed by fallow with 2.56 mg kg soil^-1 added and grain with an average loss of 0.99 mg kg soil^-1.

            Within the interaction of depth by year, the depth of 0-10 cm was highest in nitrate mg kg soil^-1 gained in 2018 and 2019 followed by 10-30 cm and 30-60 was the lowest change in both 2018 and 2019 but the loss of 2.79 mg kg soil^-1 in 2019 was much more than the loss of 0.58 mg kg soil^-1 in 2018. When looking at the interaction of depth by planting date, mid planting dates had the lowest gain of nitrates in 10-30 and 30-60 cm depths but averaged in the middle in 0-10 cm. Early planting dates were the highest nitrate change in depths from 0-30 cm but were second from 30-60 cm. The late planting date varied in its placement for each depth by being the lowest change in 0-10 cm, second highest from 10-30 cm, and highest from 30-60 cm. Depth by treatment effects were seen in the 0-10 cm depth with cover crop having an especially high nitrate gain of 12.89 mg kg soil^-1 compared to grain at 1.22 mg kg soil^-1 gained. At other depths, this difference was not as large between the treatments.

Dry Pea

            Within dry pea, depth, year, planting date, and treatment were significant effects (Table 7). Interactions of depth by year and depth by treatment were also significant. Nitrate changes decreased as depths became deeper (Table 8). 2018 averaged less of a loss of nitrate (-0.10 mg kg soil^-1) than 2019 (-0.180 mg kg soil^-1). Early planting dates averaged a lower loss of nitrate (-0.50 mg kg soil^-1) than mid (-1.12 mg kg soil^-1) or late (-2.08). The cover crop treatment had a lower average nitrate loss (-0.82 mg kg soil^-1) than grain (-1.44 mg kg soil^-1).

            In the depth by year interaction, the depths followed the trend of lower nitrate change at deeper depths with an unusually high gain of 2.44 mg kg soil^-1 in the 0-10 cm depth in 2018 compared to the loss of 0.53 mg kg soil^-1 in 2019 at the same depth. Also, there was an especially high loss of 3.46 mg kg soil^-1 at the depth of 30-60 cm in 2018 compared to a loss of 1.47 mg kg soil^-1 in 2018 at the same depth. Within the depth by treatment interaction, cover crop nitrate change was highest at depths from 0-30 cm but slightly lower (-2.97 mg kg soil^-1) than grain (-2.71 mg kg soil^-1) at 30-60 cm.

Lentil

            Depth and treatment were significant effects in lentils (Table 7). Interactions of depth by year, depth by treatment, and year by treatment were significant as well. Nitrate changes decreased as depths became deeper (Table 8). Cover crop increased nitrate by an average of 3.0 mg kg soil^-1 while grain decreased nitrate levels by an average of 1.92 mg kg soil^-1.

            In the depth by year interaction, depths followed the trend of decreasing nitrate levels at deeper depths but in 2018, the 0-10 cm depth saw an increase of 2.44 mg kg soil^-1 nitrate while in 2019, a decrease of 0.53 mg kg soil^-1 was seen. In the 30-60 cm depth, the loss of 3.46 mg kg soil^-1 nitrate in 2019 was a larger decrease than the loss of 1.47 mg kg soil^-1 in 2018 at the same depth. In the depth by treatment interaction, the cover crop treatment had a gain of nitrate mg kg soil^-1 from 0-30 cm but a loss in the 30-60 depth. The gain of mg kg soil^-1 in cover crop in the 0-10 cm depth (9.92 mg kg soil^-1) was much higher than the gain in grain at the same depth (0.09 mg kg soil^-1). When looking at the year by treatment interaction, gains in the 2019 cover crop (4.01 mg kg soil^-1) are double the 2018 amounts (2.00 mg kg soil^-1). The opposite is seen in the grain treatment with much higher losses of 3.19 mg kg soil^-1 in 2019 compared to a loss of 0.65 mg kg soil^-1 in 2018. 

Table 6: Results of a general linear model for the determination of significant effects of depth, year, crop, planting date, seeding rate, and treatment on nitrate change (mg kg soil^-1). All interactions were analyzed, and the significant interactions are listed. 

Table 7: Results of a general linear model for the determination of significant effects of depth, year, variety, planting date, seeding rate, and treatment on nitrate change (mg kg soil^-1). All interactions were analyzed, and the significant interactions are listed.

Table 8: Results of a least significant difference analysis to separate the means of significant factors on nitrate change (mg kg soil^-1) within crop type. All interactions were analyzed, and the significant interactions are listed.

Conclusion: 

The variability in precipitation between years complicated the ability to clearly see repeated differences between treatments. With this, some important effects were seen that lead to the interest in further investigation of pulse crop introduction into the winter wheat-fallow rotation in southeastern Wyoming. For example, the lack of water depletion by dry peas for the following wheat crop indicates that it is possible to introduce another cash crop option into the rotation without harming winter wheat yields. This, coupled with the N gains from the dry pea crop lead to this being the most promising crop looked at in this study. The addition of nitrogen to the system by the legume crops is very important because of the benefit of additional nitrogen in the low input cropping systems in the area.

            Even with water depletion for the following wheat crop seen in chickpeas and lentils, the addition of a new cash crop option has the potential to outweigh possible yield reductions in the winter wheat. The moisture levels in cover crop treatments is encouraging as a potential way to add flexibility in the rotation in dry years as a way to conserve water for the main winter wheat crop. Additional flexibility was seen in the lack of significant effects of different varieties, planting dates, and seeding rates as management practices across most measurements. This indicates that slightly different management practices may result in the same yields and soil attributes. This is important for ensuring the winter wheat growing season is maintained to reach maximum yield for the crop.