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 most of the 2023 growing season: all sites received 72% of normal growing season precipitation, an average of 6.87” below normal (Table 2). This followed a below average overwinter period (Oct.-Mar.), important for profile moisture recharge and early rye growth. Late April rain delayed initiation of fieldwork, and this was followed by “Flash Drought” as described by NOAA: 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 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.

Relationship between termination timing, weed suppression, and soybean yield

The generalized treatment relationships are shown in Figure 4 and have two summary phases: adding rye to the no cover system with PP termination resulted in 41.5% GTRW suppression and a 2.0% yield increase; subsequent termination dates increased GTRW suppression in a nonlinear fashion to a maximum of 87.5% at anthesis while soybean yield declined nonlinearly, accelerating to a reduction of 24.7%. Weed suppression on the extremes of termination timing are lower than we measured in Project ONC21-094 while yield effects are similar and we attribute this to the “flash drought” conditions of  May-June, discussed below.

Shifting the comparison to termination timing eliminates the phases and demonstrates the relationships relative to PP termination: suppression increases in a nonlinear fashion, accelerating to the maximum at or near anthesis while yield declines in a near linear fashion. Figure 4 generalizations represent the actual responses well, even without time scaling. Individual variable responses follow.

 Rye AGB

Rye AGB accumulation is presented in Figure 5, combined over sites to estimate the general response and by location due to site x termination timing interactions (p<0.001). Rye data is presented in measured units rather than relative to examine the dosing effect on GTRW suppression and yield. Sites 2,3 experienced similar biomass accumulation trends and final biomass yields (3,009 and 3,445 lb./a respectively) at anthesis. Site 1 produced less AGB at each termination, accumulated it between terminations at a lower rate, and produced a final accumulation (1,274 lb./a) which was 39.4% of other sites. This site, depicted in Figure 2, was the most well drained and rye began exhibiting moisture stress by late may (VE+7). This site also produced the greatest mean soybean yield; this interaction will be examined in the yield discussion. Of note, S3 had the greatest final AGB accumulation (3,445 lb./a) yet was planted 35 days later than the other sites due to corn harvest delays. Sites 2,3 performed similarly through anthesis yet interacted at various sampling intervals. This was due to differential rainfall events and precipitation amounts creating periods of accelerated growth. Rye canopy (% living green cover) was similar (8.4%) at the first termination, yet S3 exceeded the other site at subsequent measurements (data not shown). The relationship between S2 and 3 supports the widely held view within the regenerative agriculture community that rye can be planted later than traditional wisdom would suggest (very late) and still produce benefits.  

GTRW suppression

The relationship between termination timing and GTRW population reduction is shown in Figure 6.

This data presentation shows suppression relative to the no rye control (upper, beginning at 41.3%) and the change in suppression relative PP to termination (lower). The second analysis is important from a management perspective because it indicates the gain in suppression by delaying termination which can be weighed against potential yield impacts to arrive at the termination decision.

Both relationships are quadratic, increasing the rate of suppression as termination is delayed. Both also indicate a relatively flat (unchanging) rate of suppression until 7-14 DAE when 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 5, relative data not shown). Our practical observation is that we did not experience appreciable suppression from PP until 25 days later: termination should not be delayed if yield impacts are expected.

Sites differed in their respective weed pressures (populations as measured in control) but not in their relative suppression relationships (p=0.282). Sites 1,3 had greatest GTRW populations measured at time of post emergence herbicide application (2,568 and 7,615 plants/acre) while S2 was extremely low, 51 p/a. Marestail predominated S1, followed by waterhemp, S3 had a mix of giant ragweed and marestail while S3 had all 3 species. We assume the residual herbicide was fully activated: application was made to moist soil (0.44 and 0.57” of precipitation to sites within 2 days prior) and 0.89 and 0.97” within 14 DAA, well in excess of manufacturer and regional Extension guidelines. Post application precipitation was also sufficient to wash intercepted products from rye tissue, so these measured population densities are the result of normal (and expected) residual herbicide breakdown and therefore, reductions in GTRW populations are weed suppression by rye.

Yield

The relationship between termination timing and yield reduction, expressed as relative yield is shown in Figure 7, and presented in the same format as weed suppression. As with weed suppression, the full range of termination timings results in a quadradic response: a 5.3% yield decline if termination is delayed until VE followed by an increasing rate of yield decline. 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.

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 sake of interpretation, we regard the magnitude of increases to outweigh the decrease, therefore report 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.

Other findings

Staging: Soybean development, based on growth stage synchronized well with weekly intervals in 2023. This did not appear to be the case at the VE+14 samplings 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 will continue comparing these two methods of staging in Cycle 2. We also characterized rye development, measuring height, leaf number and head height (measured to the approximate center) above the soil surface during stem elongation. None of these measures appeared consistent in the field so this data remains unanalyzed at the time of this report, but we will repeat them in Cycle 2.

Delayed maturity: Delaying termination delayed soybean maturity as indicated in Figure 8. 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 were such that we only detected moisture differences at S3 and only the anthesis termination.

Relationship between rye AGB, weed suppression and yield decline: While outside the scope of this project, there appears to be a strong relationship between these variables. This will be examined in the final report.

2023 Lessons Learned

Terminate rye early under drought conditions, either before planting or immediately after, allowing the combination of termination and residual herbicides in one application. Our data shows a yield decline with termination timings after preplant but the rate of decline is low initially, so impacts are negligible at planting.

Early termination captures nearly 50% of the total weed suppression, thus representing the “sweet spot” in a dry year. Delaying termination to maximize weed suppression resulted in a severe yield penalty.

Stay the course: The properly managed cover crop resulted in a mean 1.98% yield increase. In our trials, having the growing rye helped manage soil moisture from late April rains, producing suitable planting conditions on both silt and sandy loams. Many farmers expressed frustration with cover crops in 2023: difficult planting conditions and yield reductions. Our results demonstrate careful attention to management detail can provide positive results.