In recent years, there has been renewed interest in early soybean planting dates. Much of that interest is based on relatively recent work done in Nebraska, Iowa and Indiana. In Kansas, planting dates and variety maturity combinations are often chosen to avoid having soybeans flower or fill seed during the most stressful times of year. Does early planting achieve this under Kansas conditions? If you planted your soybeans in late April or this first week of May, did you do the right thing? If you still have soybeans to plant, how long should you wait?
First, we should review the research results.
Recent research in other states
In the Nebraska research, yields of irrigated soybean decreased somewhat when planting was delayed past May 1. This research was done at one location, for two years. The early planted beans had more pod-bearing nodes. It should be noted that with irrigated soybeans, the potential for adverse environmental conditions during bloom and grain fill is greatly reduced. In these trials, a fungicide and insecticide seed treatment was used to minimize the effects of planting early into cool, wet soils.
In Iowa, the research was done under dryland conditions – but dryland in Iowa is not quite the same as dryland in Kansas, except possibly northeast Kansas. The research done in Iowa, consisting of 24 tests, found that yields were higher 79% of the time when soybeans were planted in late-April or early May than when planted about May 20. The greatest response to early planting was in high-yield environments.
In Indiana, research at one location in 2006 and 2007 also found that soybean yields were higher when planted in late April or early May than at later dates. Earlier planting was associated with more pods per unit area.
The most recent research to examine planting date effects was conducted in north central and northeast Kansas. It was embedded in a study funded by the Kansas Soybean Commission designed to determine the relative importance of seed treatments at early and late planting dates. Results from this series of studies conducted in 2009 and 2010 indicated a 2 to 9 bushel yield reduction when planting was delayed from late April or early May to early June. The advantage for early planting was greatest with treated seed (Figure 1).
A one-year study conducted at Manhattan in 2010 as a complement to the above experiment added more planting dates. Results in that year showed a consistent 0.22 bushel/acre/day reduction in yield as planting was delayed from late April through early July (Figure 2). In reality, the yield reduction was slightly less than that through May and slightly greater as planting was delayed into late June and July. This is a limited data set so it may not be predictive over a wide area or for many years, but it agrees with other work that has shown reductions in yield as planting is delayed in northeast and north central Kansas.
The previous major round of research studies in Kansas looking at planting dates, was in the early 2000’s, which was a period of generally very low yields. The basic conclusions varied by region of the state:
North Central/Northeast: At Topeka, with yields up to 55 bu/acre, yields increased with planting dates in late April compared to May for Group III and IV varieties. In Belleville, with yields up to 50 bu/acre, yields were generally unaffected by planting dates up to mid-May, but had a fairly sizable yield drop when planting in June. In Powhattan, with yields less than 30 bu/acre, there was no advantage to planting any earlier than early May.
* East Central: There is no evidence to support earlier planting.
* Southeast: There were no tests conducted in southeast Kansas during this round of studies.
* South Central: At Hutchinson and Wellington, yields were low but generally maximized by planting in late April.
* Western Kansas: Yields were very poor during the years of these studies. Planting dates had no effect on yields.
Conclusions and recommendations
* In northeast Kansas planting dates should be as early as practical. Late-April planting would be advisable if soil conditions allow. If you have already planted soybeans in this area of the state, research would support your decision, especially if the seed was treated with a fungicide and insecticide. Varieties with resistance to soybean cyst nematode and sudden death syndrome should be used if possible when planting early. Do not plant into soils that are too wet, however. Also, do not plant until soil temperatures are at least 55 to 60 degrees. If planted into soils cooler than that, seedlings may eventually emerge but will have poor vigor.
* In drier areas of Kansas and on shallow soils, research results would suggest that yields are most consistent when planting soybeans in late May to early June. By planting in that timeframe, soybeans will bloom and fill seed in August and early September, when nights are cooler and the worst of heat and drought stress is usually over.
* Ultimately, weather patterns dictate soybean yields, especially under dryland conditions. There is no guarantee any certain planting date will always result in the greatest soybean yields in Kansas.