A UVM blog Out Croppings: Important crop news from the field!

Although it hasn’t felt like it, fall is right around the corner. Despite dry conditions through much of the growing season, and still persisting in some regions, soybeans seem to be doing well. Overall, they look tall and healthy with pods filling and not too much disease. Since it will be a while longer until we have all the results from our 2021 trials, here are some highlights from our soybean work in 2020 to consider while you wait. We hope you find this information useful in making informed decisions on your farm. Full results of research trials can be found at uvm.edu/extension/nwcrops/research. Reach out to us if you have any questions (cropsoil@uvm.edu)! 

Variety Selection

In 2020 we trialed 19 conventional and 8 organic soybean varieties spanning maturity groups 0-2. Despite exceptionally dry and hot weather, soybeans in both trials performed well averaging 65.3 bu^-1 and 55.8 bu ac^-1 in the conventional and organic trials respectively (Figure 1 and Figure 2). However, as you can see from the figures below, there was quite a range in yield between varieties. Interestingly, in both trials we observed high yields from short and long season varieties indicating that high yields can be attained in our northern climate. It is important to consider such trial results when selecting a variety that fits your farm’s goals and is well suited to its environment. Check out the full reports on our website for more details. 

Cover Cropping with Soybeans

In 2020 we conducted two cover cropping trials with soybeans. The goals of the trials were to: 

1) investigate the impact of various cover crop species and mixtures on subsequent soybean yield and quality, as well as nutrient and soil health dynamics, and 

2) investigate the impact of termination methods on soybean yields and quality. 

Introducing a cover crop into a cropping system can provide a multitude of benefits but also presents challenges. Cover crops used in this region can be categorized into two main groups: winterkilled and overwintering. While overwintering cover crops provide protection to the soil through the winter and regrow the following spring, having them regrow in the spring requires terminating them in a timely manner without delaying planting impeding nutrient availability for the cash crop. In 2017, we saw a 7.5 bu ac^-1 yield reduction when soybeans followed winter rye but no yield reduction when soybeans followed a winter killed cover crop. Interestingly the experiment was repeated from 2018 through 2020 and although the soybeans yields were often higher following a winterkilled cover crop, it wasn’t statistically different in yields compared to soybeans following winter rye (Table1). This suggests that utilizing an overwintering cover crop that was incorporated into the soil prior to seeding soybean, does not result in a significant yield reduction. We are still trying to understand why the overwinter cover crop might lead to yield reductions in soybeans. 

When comparing the different cover crop treatments, there were differences in soil nitrate concentration throughout the season (Figure 3). You can see that incorporating the overwintering cover crop biomass (winter wheat and triticale) into the soil resulted in the lowest nitrate availability throughout the season whereas the winterkilled cover crop such as annual ryegrass and tillage radishes, led to significantly higher soil nitrate concentration over the season. The additional nitrogen in the spring may help the soybeans establish and could potential lead to higher yields.   

We also compared three different termination methods for terminating overwintering cover crops:

1. Tillage 

2. Pre-plant herbicide application 

3. Post-plant herbicide application (planting green) 

Figure 4 shows the spring cover crop biomass and subsequent soybean yield for each of the termination treatments. The pre-plant herbicide application treatment (pre-spray) saw the lowest cover crop biomass and the highest soybean yield of over 70 bu ac^-1. In the tillage treatment, although the cover crop biomass was significantly higher at over 2 tons ac^-1, no significant soybean yield reduction was observed. The lowest soybean yield was observed in the post-plant herbicide application treatment where yields were almost 30 bu ac^-1 lower than the tillage and pre-spray treatments. This treatment also experienced significantly lower soil nitrate and soil moisture concentrations compared to the tillage treatment over the season which may have negatively impacted soybean establishment and performance. 

Planting Dates

Over the last four years we have also been conducting research on altering soybean planting dates in order to better understand the best range of planting dates and their subsequent impact on soybean performance in our region. Two-row plots of an early group 1 maturity variety and a mid-group 1 maturity variety were planted approximately weekly from 14-May through 2-Jul. 

The significant interaction between relative maturity and planting date for yield indicates that soybeans of different maturity groups have different yield responses to delaying planting dates (Figure 5). We would expect shorter season varieties to begin to out yield longer season varieties as planting dates are delayed. However, that is not what we observed in this trial. Although we did see the later maturing variety out yielding the early maturing variety in early planting dates, both varieties experienced significant yield declines as planting dates were delayed beyond mid-June and the early maturing variety did not outperform the late maturing variety at these dates. This indicates that, even for shorter season varieties, delaying planting until late June or later will have a significant impact on soybean yields. This was likely impacted by the early frost that negatively affected both maturities despite adequate GDDs. The extremely low yields experienced in the first two planting dates was likely due to an error in herbicide application that contributed to damage to early planted treatments, not a factor or the planting date itself. This is further evidenced by growth stage data collected throughout the season that shows the first two planting dates aligning with the growth stages of soybeans planted 3-4 weeks later. 

Soybean yields ranged from 1519 to 3411 lbs ac^-1 or 25.3 to 56.9 bu ac^-1 with the highest yields being obtained when planting between 28-May and 19-Jun (Figure 6). However, the first two planting date yields were likely negatively impacted by an erroneous herbicide application. These data suggest that delaying planting to late June and beyond negatively impacts soybean yields in this region. However, some of the later dates may not support such high yields in years where weather conditions are less conducive to soybean productivity.  

Soybean yields were significantly impacted by planting date with the highest yields observed when soybeans were planted between late-May and mid-June. These data suggest that delaying planting of soybeans beyond this is likely to result in depressed yields. An erroneous herbicide application likely impacted the first two planting dates. There was no significant difference in oil content between planting dates. Soybean yield was not significantly impacted by relative maturity of the variety as both varieties were able to reach maturity and produce high yields. However, these trends may not hold in years with more normal GDD accumulation.