Finding the sweet spot: rye termination timing to balance weed suppression and yield reduction in green planted no-till soybean.

Growing season summary

Drought conditions influenced the 2023 growing season. We received 72% of normal growing season precipitation and accumulated a 6.87” deficit (Table 2) following a dry overwinter period (Oct.-Mar.) which contributed minimally to soil profile moisture recharge and support of early rye growth. Late April rain delayed initiation of fieldwork, and this was followed by “Flash Drought” as described by NOAA: two changes in U.S. Drought Monitor status within a 30-day period. May and June precipitation was 38.8% of normal, the Palmyra area (Sites 1,2) was drier with 3.06” (-6.33” LTM) while East Troy (S3) received 4.24” (-5.17” LTM). This differential partially explains rye AGB production differences between the two areas, discussed later. This is also the critical period for interactions between growing rye, soybean, and GTRW and likely had a major impact on weed suppression and soybean yield. The August- September period also had differential precipitation, critical in August during grain fill. East Troy received 1.61” less rainfall and missed a late August event which would have helped increase seed size and thus yield. Season total growing degree day (GDD) accumulation was near normal and had no impact on results.

The 2024 growing season was the opposite extreme, most influenced by a five-month period of consecutive above normal monthly precipitation beginning in March which heavily influenced the rye/ soybean relationship. Frequent rainfall disrupted field operations and caused the asynchrony in planting dates between Palmyra and East Troy sites but ultimately resulted in near ideal growing conditions into August and no observed moisture stress in soybean treatments with later rye termination. As in 2022-23, overwinter precipitation was below normal through February which contributed to continuing drought conditions but March-April rains (>11” total) recharged the soil profile. On the other end of the growing season, precipitation became severely limited during August and the region developed “flash drought” conditions by early September. General trial effects included forced maturity (early soybean death with leaf retention) at Palmyra Sites 5,6 and overall yield and test weight reduction despite the potential for an exceptional growing season and high yields that could be expected from a growing season with a 4.53” rainfall surplus. Growing degree day accumulation was 104% of normal.

Annual cropping cycle (Oct.-Sept.) precipitation trends, presented as cumulative departure from normal, are shown in Figure 4. This method of data presentation shows near mirror images of cumulative surplus/ deficit precipitation, especially from March onward and explain much of the differential performance observed in our trials.

Relationship between termination timing, weed suppression, and soybean yield

The generalized relationships are shown in Figure 5. The intent of this presentation is to introduce the data , comparing the effects of contrasting growing seasons and to characterize the two distinct phases of our treatment arrangement: the no rye control compared to PP termination which represents the addition of rye to the no-till soybean system and also serves as the reference point for subsequent analysis; and the effect of delayed termination. As such, it captures the relationship but not time-scale effects which will be discussed below. In general, the relationship trends are similar regardless of growing season conditions: The inclusion of rye results in weed suppression which increases with delayed termination in a non-linear fashion, with rapid acceleration beginning between 7-14 days after soybean emergence. Inclusion of rye also results in a yield increase of 1.6 to 2% (to the 100% of maximum level, making it the reference point for subsequent analysis) which then declines with delayed termination at a rate determined mostly on precipitation and soil moisture. This data continues to make a compelling argument for cover crop use, combining the benefits of 41 to 82% weed suppression and the yield increase. Subsequent analysis will shift to focus to the impact of delayed termination, using the PP termination timing as a reference point for individual responses.

GTRW suppression

The relationship between termination timing and GTRW suppression is shown in Figure 6. Both years follow the general trend described above but differ in their relative initial level of suppression, initial gains or losses in suppression from PP termination and their final level of suppression.

In 2023, suppression levels began at 41.4% and increased with time in a nonlinear fashion, achieving the maximum 87.5% level of suppression at anthesis. This pattern of suppression increase very closely followed that of rye AGB accumulation (Figure 7) and the relationship between the two was conclusive (p <0.001).  Because of the nonlinear pattern and acceleration with time, most of the gain in suppression occurred later, after V1 (25 days after first termination (DAFT), 60% suppression). To better gauge when relative gains in suppression are maximized, we reanalyzed the data, adjusting the absolute range of 41.3 to 87.5% suppression to relative percent (0-100%) to estimate the change (gain) from the PP termination reference point. In this analysis we found a relatively flat (unchanging) rate of suppression until 7-14 DAE at which point it increases markedly. When isolated, this interval exhibited a 5.3% increase/ day (p=0.181) and corresponds with a change in AGB from 51.1 to 67.9 % (% of maximum, Figure 8, relative data not shown). Our practical observation is that we did not experience appreciable suppression until 25 days DAFT and that termination should not be delayed if yield impacts are expected.

In 2024, we observed a combined 82.3% suppression with the PP termination and maximum suppression, 95.1% at V2 (28 DAFT, ~ 7 days before anthesis). In the combined analysis, only 12.8% additional suppression (13.4% of the total suppression) was gained by delaying termination from PP and we observed a reduction in suppression for VE and V1 staged terminations. We attribute this reduction to spray interception of the residual herbicide by green rye biomass: PP terminated biomass was browning and desiccated at the time of application reducing interception and improving herbicide efficacy while greater biomass levels with later termination compensated for loss of herbicide efficacy. As in 2023, the pattern of rye AGB production (Figure 7) matched that of suppression (Figure 6) with a tendency (p=0.119) for AGB amounts to influence suppression.

In 2024, we also observed differential site responses for suppression (Figure 8) which require examination because they temper the above discussion but also help clarify the relationship between AGB and suppression. Site 7 (ET on the graph) had lower initial AGB (483 vs. 285 lb. DM/acre) due to later rye planting (23 days) and thus lower initial suppression. This site also exhibited the slight reduction in suppression for the second termination date (VE, please note that the 7-day lag in VE application dates was due to a planting delay discussed in methods) but then exhibited an accelerated gain in suppression related to compensatory rye growth and AGB production (Figure 9). Sites 4-6 exhibited similar responses and combined, achieved 93.0% suppression with PP termination, exhibited the spray interception reduction followed by a slight increase in suppression which we consider marginal.

When sites are combined, we suggest that the large annual difference in initial suppression, 41.4 vs. 82.3% is due to differential rye AGB at PP termination, 202 vs. 433 lb. DM/ acre, a 2.14-fold increase in 2024. This observation has major implications as we consider management options and will be explored after yield effects are discussed.

Yield

The relationship between termination timing and yield reduction, expressed as relative yield is shown in Figure 10, and presented in the same format as weed suppression. The PP treatment resulted in maximum yield in both years so represents 100% relative yield.

In 2023, drought conditions limited yield overall and delaying termination beyond PP resulted in a nonlinear yield loss which accelerated with time to 26.5% at anthesis termination. Yield was reduced 5.3% when termination was delayed until emergence after which the rate of decline accelerated. When reanalyzed, shifting the reference to VE, yield reduction becomes linear at rates of 1.07%/ day or 0.54 bu./acre/day under the yield potential conditions 2023. We also estimated a 1.7% reduction at 3 days after planting (DAP), an important benchmark considering that many residual herbicide labels require application by then and it may be desirable combine this application with rye termination for efficiency reasons. In 2023, we experienced some minor differential site responses which do not affect our findings or recommendations but are significant enough to discuss below in the endnotes.

In 2024, delaying termination also reduced yield but to a lesser extent and at a nearly constant rate for an apparent reduction of 3.6% at final termination, calculated from individual site means. However, a differential site response skews this relationship, resulting in a slight underestimation of yield reduction at the anthesis termination timing when sites are considered together. Figure 11 shows the differential relationships: three of the sites exhibit a continuing yield decline to a final yield at 91.2% of PP, yield at Site 4 increased over the later termination dates and achieved a final yield at 99.7% of PP. We attribute this to greater mid to late-season soil moisture retention and the ability of the crop to reach maturity normally at this site during the late season “flash drought.” Of the 2024 Palmyra sites, S4 had the greatest moisture storage potential based on its silt loam soil type and low landscape position compared to the other, upland sites. Here, the additional AGB accumulation resulting from later termination probably reduced evaporative soil moisture loss, conserving enough moisture to allow normal maturity while the lighter soil sites did not, reducing yield. Thus, our observation is that 2024 yield reduction was more variable, impacted by inherent site characteristics and for discussion purposes, we conclude a range of yield impacts from none to an approximate 9% reduction.

Rye AGB: weed suppression relationship

This relationship is shown in Figure 12, all data (upper) and by year (lower). We evaluated the relationship to help explain trial results, knowing that most work in this field suggest that the level of suppression is a dosing effect, directly related to AGB production and for some cropping situations (organic for example), Extension Weed Scientists have established threshold minimums for weed suppression efficacy. The proposed mechanisms of residue suppression: physical barrier to weed emergence (the mulch effect), light interception to prevent weed seed germination, and buffering of soil temperature, preventing fluctuations which trigger germination in some species (most notably waterhemp) would all require greater levels of biomass for increased efficacy. Our goal was to seek information which could possibly be used in-season to guide the termination timing decision, given the potential for yield reduction by delaying termination: how much suppression are we giving up protecting yield?

The combined analysis (R² = 0.299) suggests a threshold value of 3,727 lb. DM/acre (calculated) to achieve maximum suppression but this value is questionable given the limited amount of AGB data above 2,000 lb./acre and the relatively poor fit of the equation to the data, especially at lower DM values.

The analysis by year (year, p <0.001) improves the overall data fit and results in linear relationships and rate constants which are consistent with a dosing effect. Also important are the relationship between the initial starting point (level of suppression) and the rate constant. In 2023, we began with 41.4% suppression and gained 1.9% suppression for each additional 100 lb./acre of AGB added. In 2024, initial suppression was doubled at (82.3%) and related to increased mean AGB (433 vs 202 lb./acre, a 2.1-fold increase) but had a lower rate of suppression gain, 0.9%/ 100 lb./acre. All things being equal, this relationship could potentially be factored into the termination timing decision to protect yield if the relationship holds true: years with lower AGB before planting will have lower initial suppression but exhibit greater rates of gain while the inverse is true for years of greater AGB before planting. This will be discussed further later.

Even with better model fits for the annual analysis, the data is poorly fitted in the lowest range of AGB and in several cases, the level of suppression far exceeded the model. Given the understanding of the mechanisms of suppression discussed above, it is questionable that the limited AGB available in these cases could be contributing to suppression. Figure 13 illustrates this point. The upper photo depicts PP terminated rye residue at VE in 2023. We achieved a mean suppression of 41.4% yet the desiccated residue provides little cover to support the reported mechanisms responsible suppression and in fact, the residue disappears by the time of weed population characterization/ post emergence herbicide application 35 to 40 days later. Could allelopathy (inter-specific chemical inhibition) be responsible for a portion of the suppression? The lower photo in figure 14 depicts soybean plots in mid-August in our predecessor project (ONC21-094), containing the no rye treatment on the right, the PP termination treatment on the left and both PP and anthesis terminations in the background. It is evident from the lack of both rye residue and late-season grass biomass in the PP treatment that some form of suppression unrelated to AGB is occurring. If real, could this phenomenon also be used to guide the termination timing decision? We have now observed it over 14 site-years spanning 4 different growing seasons and suggest it is evidence of allelopathy whose effect could be maximized by maximizing AGB at PP termination.

Other findings

Staging: Soybean development, based on growth stage synchronized well with weekly intervals in both years. This did not appear to be the case at the VE+14 samplings under conditions of moisture stress in 2023 but rainfall within a day of sampling rehydrated plants, which showed they were at the V2 stage: tissue had differentiated, the cells needed water to expand. We also characterized rye development, measuring height, leaf number and head height (measured to its approximate middle within the stem) above the soil surface during stem elongation at termination. None of these measures appeared consistent in the field or between years so we consider soybean growth stage a better benchmark for staging the termination decision.

Delayed maturity: Delaying termination delayed soybean maturity as indicated in Figure 3 (right). This is consistent with our findings in Project ONC21-094 where termination at anthesis delayed maturity by 7 days and resulted in greater grain moisture at harvest. Preharvest conditions and forced maturity were such that we only detected grain moisture differences at S3 in 2023 and then, only for the anthesis termination but this could become a harvest management consideration in years of normal soybean maturation.

Lessons learned and management recommendations

Preplant termination was the “sweet spot” in 2023, protecting yield while capturing nearly 50% of possible total weed suppression. Delaying termination until soybean emergence reduced yield 5.2% and further delay caused a nearly 1.1% reduction per day. The major acceleration of suppression occurred 7 to 14 days after VE, but yield was reduced to a minimum of 7.7% by then, representing a severe yield penalty to capture the additional suppression gain.

In 2024, the exact “sweet spot” is less obvious but occurred with termination at or before emergence. Preplant termination protected yield potential and created a gain in suppression by apparently reducing residual herbicide spray interception which reduced the overall level of suppression until offset by greater AGB with termination timings between V1-2 depending on site. We estimate no yield reduction if termination occurred within 3 DAP and a minimal (0.7%) reduction if delayed until emergence, but these timings resulted in suppression reduction. Further delay mostly resulted in yield loss.

Based solely on the relationship between weed suppression and yield, the termination decision should be based on soil moisture conditions and prevailing weather patterns before planting. In dry years, our experience conclusively demonstrates rye should be terminated before planting, even if it requires an additional application operation. In years of ample moisture, the urgency to terminate is reduced, easily allowing planting green and the possibility of combining termination and residual applications shortly after planting. Delaying termination for other considerations related to soil health (continuous living roots or increased biomass) should be weighed carefully against potential yield penalties and 10-day and long-term weather outlooks are critical in this decision. We experienced “flash droughts in both study years which demonstrated how quickly conditions can change.

Finally, we suggest that rye should be managed for maximum AGB yield before a potential PP termination to maximize initial suppression and to take the risk out of the decision. We observed a doubling of suppression with a like increase in AGB as well as reduced initial suppression when later planting limited AGB at individual sites. Earlier planting practices, such as following directly behind the combine rather than waiting for the end of harvest to begin planting would aid timely establishment and help avoid potential weather delays. Adding modest amounts of early season N has also been suggested to increase AGB before planting in Wisconsin and is a practice our group plans to investigate. Although intuitive, we discount increasing seeding rate for this purpose based on Project ONC21-094 results where doubling the rate to 80 lb./acre only increased AGB DM 34% on average and, as in this project, earlier rye planting had a greater influence on DM levels at PP termination.

Endnotes

2023 Yield: As with weed pressure, sites differed in the absolute yield (S1 > S2 > S3, the Palmyra sites benefited from differential late season precipitation discussed above) and displayed two interactions with termination timing (p=0.011) which deserve examination. The first, which leaves the regression analysis unaffected is the comparison between no rye and PP termination: S1,3 experienced yield increases of 3.6 and 3.4% respectively while S2 had a -1.02% decline. For the sake of interpretation, we regard the magnitude of increases to outweigh the decrease, therefore reporting a 1.98% yield increase to cover. The second interaction is a divergence of S2 from S1 beginning at the VE termination timing. Reanalysis of the data using VE as the starting point minimizes the ANOVA interaction term (p=0.117) allowing us to combine sites for a single response function and more importantly, a universal estimate of yield decline.