Pulse Beat Individual Articles

Dry Bean Responses in a Droughty Year: Findings from Tillage and Nitrogen Fertility Trials

Megan Bourns, MSc, Agronomist – On-Farm Network, MPSG – Fall/Winter (December) Pulse Beat 2021

The 2021 growing season was full of production frustrations and environmental challenges, but it led to some interesting findings from On-Farm Network (OFN) dry bean trials. There were fantastic visual differences between treatments throughout the season and yield response at harvest in both the dry bean tillage and nitrogen fertility trials. Although these responses were found in a year with sub-optimal growing season conditions, the findings provide valuable insights into dry bean production and considerations for future management decisions.

Dry Bean Tillage

The 2020 season was the OFN’s first go-around investigating tillage practices for dry bean production, comparing strip-till and conventional till for pinto beans. The main lesson from the first year centred around the soil protection benefit of strip-till, compared to more aggressive conventional tillage. There were some big wind events in early June of 2020 that led to enough soil blowing and sandblasting of the pinto bean seedlings that re-seeding was necessary in a large portion of the conventional tilled areas of the trial. Seedlings in the strip-till plots were not affected to the same extent.

In addition to the soil conservation aspects of a reduced tillage regime, moisture management is another obvious consideration — and a timely one, given the conditions of the 2021 growing season. The OFN hosted two strip-till vs. conventional till trials this season, one in pinto beans and one in black beans. Both trials were in south-central Manitoba on clay soils. Trials were established with tillage last fall, both on wheat stubble.

Early season observations from both tillage trials were uneventful until the transition between vegetative and reproductive growth stages. As flower buds and vines formed, the conventional tilled beans were visually smaller and less vigorous than the strip-till beans. We observed this growth difference in both the pinto bean and black bean trials, but the differences remained very persistent and stark at the pinto trial. In fact, the conventional tilled pinto beans were set back and persistently green to the point of being unable to harvest those plots. As a result, the conventional till beans were tilled under and yield was effectively zero for those plots.

Just a few miles away at the black bean trial, differences in growth and vigour between tillage treatments were evident as the beans moved into flowering, but they were much less obvious than in the pinto bean trial. The most notable difference was in the extent of vine production, where the strip-tilled beans had more vine growth than the conventional tilled beans. At this trial, beans from both tillage treatments progressed through maturity and dry-down as expected and we were able to harvest as planned. The yield difference between tillage treatments was significant, with strip-till beans yielding 290 lbs/ac more than conventional tilled beans (Figure 1).

Looking Ahead – The Next Questions

So, we know reducing tillage in a moisture deficit year can influence dry bean growth and yield. One of our questions is, how much of a difference did tillage regime make in soil moisture throughout the season? In addition to visual observations and yield, this year, we tracked soil moisture throughout the growing season to a 1 m depth in both tillage trials. Data analysis is ongoing, and results will be available soon!

Dry bean tillage trials will continue if there is farmer interest. We have captured dry bean response to strip-till in two dry years. Investigating this question under different growing season conditions may help flesh out where and when strip-till is a beneficial practice for dry bean production.

Dry Bean Nitrogen Fertility

Investigation of dry bean response to nitrogen (N) in Manitoba has been ongoing since 2017. The Soybean and Pulse Agronomy Lab at the University of Manitoba conducted small-plot research investigating pinto and navy bean yield response to spring broadcast. It incorporated N fertilizer at rates ranging from 0–140 lbs N/ac. They found a significant yield increase only with the highest rate of nitrogen; however, there was no significant difference between economic N return for any N fertilizer treatment.

These small-plot findings generated interest in exploring N fertilizer strategies for dry bean production on-farm, at a full field scale. The OFN began dry bean N fertility trials in 2019. To date, five trials have been conducted, investigating some combinations of fertilizer rates ranging from 0–140 lbs N/ac in pinto, navy and black beans. Until 2021, field-scale results reflected small-plot findings — that there were no significant yield increases with N fertilization compared to the control, regardless of N rate or bean type.

Each growing season of the small-plot and field-scale trials had been drier than normal. So the question about the effect of N rate on dry bean yield persisted — in a year with more “normal” precipitation and higher yield potential, would those N rates make a difference in yield? With that question in mind, we set up another trial in 2021 comparing rates of 0, 35 and 70 lbs N/ac (spring broadcast and incorporated urea) in pinto beans. Our goal of testing this question under more “normal” moisture conditions obviously did not play out as planned; however, our findings are quite interesting and may shed some light on the lack of yield response in other years.

Similar to the dry bean tillage trials, our observations noted similar growth and vigour among N fertilizer rate treatments until the transition to reproductive growth stages. Once flower buds started to form, it was evident that the 0 N control strips were less vigorous than the N-fertilized strips, and these differences persisted through the R stages. Not surprisingly and complementary to the visual differences in vigour throughout the season, pinto bean yield response to N fertilizer rate was significant. The 70 lbs N/ac treatment yielded 150 lbs/ac more than the 0 N control (Figure 2). The 35 lbs N/ac treatment yielded statistically similarly to both the 0 N control and the higher rate (70 lbs N/ac) of N fertilizer.

So, why did pinto bean yield respond to N fertilization in this dry year? The answer is not likely simple, but some supplemental data we collected this season may shed some light. In addition to fertilizer-N, soil nitrogen and nodule formation can contribute to dry bean N nutrition. Typically, nodulation ratings tend to decline with increasing N fertilizer, as we would expect. This season, nodulation ratings were “fair” in the 70 lbs N/ac treatment and “good” to “excellent” in both the 0 and 35 lbs N/ac treatments. This is consistent with findings in 2019 and 2020. So, what was different about the 2021 season that led to an increase in yield with the higher rate of N fertilizer? Perhaps mineralization or lack thereof holds the answer.

This season, as a data collection add-on to the dry bean N trials, we established microplots in each of the 0 N check strips of the trial, where we soil sampled every couple of weeks to a depth of 12 inches. The intent was to track nitrate over the course of the season. As we know, nitrate levels will vary as the season progresses and mineralization is one of the processes governing this temporal variability. Mineralization is the soil process whereby microbial or organic N is transformed to mineral, plant-available N and this can contribute to crop N-nutrition in-season. Interestingly, this season, our microplot sampling revealed very consistent nitrate in the top 12 inches from the middle of June through to the end of the season (Figure 3).

Mineralization requires heat and moisture, and the latter was obviously in short supply for the majority of the 2021 growing season. Limited moisture and limited mineralization could have reduced the supply of soil-derived nitrogen for the dry beans this season compared to other years, increasing the demand for fertilizer-N. Perhaps, this played a role in the significant yield differences we saw this season. However, further investigation into the dynamics of dry bean N supply is required.

Looking Ahead – The Next Questions

Dry bean yield response to N fertilizer has not been common in small-plot or on-farm trials. The question that remains is, why? Dry bean N nutrition is a dynamic combination of soil-derived N, fertilizer-N and potentially nodule-N. Further understanding of these three N sources and their interactions will aid in our ability to predict where and when N fertilizer is required and at what rates response is likely. This will require more dry bean N fertility trials, with increased data collection of nodule formation, soil-N dynamics through the season and yield response in different growing season conditions. The OFN will continue dry bean N trials with interested farmers!