Smaller soybean plants caused by cool weather

Some growers have observed that there appears to be a smaller growth of soybeans than normal, and are concerned that it could result in reduced yields. Because of this, some are also wondering if the use of foliar fertilizers or fungicides could help make up the difference.

Indeed, many soybeans were planted late locally and across most of the state. According to the latest weekly survey and published Crop and Weather Report from National Agriculture Statistic Service, soybean flowering is at 35 percent, behind the five-year average of 47 percent in Kentucky. Most farmers’ internal clock says that soybeans should be larger by now. Most years, that is correct. This is not most years. As many have observed this summer is cooler and wetter than most years, especially recent ones.

As soybeans are getting to flowering (growth stage R1) they may be a little smaller than in previous years. The cooler temperatures combined with later planting dates will cause smaller plants. The smaller plants could be a concern if rows are not closed in shortly after flowering. If the soybean rows are closed, then height is less of an issue. As long as the rows are closed, tall plants do not automatically equal high soybean yields.

The situation is progressive. If the rows are not closed and the soybeans begin to flower, then yield potential is likely lost. As the soybeans move into pod development and the rows are not closed, yield potential is even more likely lost. If the soybeans get to seed fill and the rows are not closed, yield potential is lost.

This brings us to the main question: will a foliar fertilizer or a foliar fungicide help? The short answer is probably not. Fungicides will not improve the speed at which soybeans grow and will not help with canopy closure, in the absence of a disease. Fungicides will help soybeans retain leaves, if a disease is present in the field. However, the cooler night temperatures and the smaller soybean plants both contribute to less of a threat from diseases this season.

Foliar fertilizers will not compensate for lower temperatures. They will not increase the speed of growth, assuming phosphate (P2O5) and potash (K2O) levels are adequate in the field. They will make the plants greener and that might make someone feel better.

If you or your neighbor is absolutely set on spraying something, then consider the foliar fertilizer. It likely will make the plants greener and it should cost a little less than the fungicide. Or, take that money you would have spent on the foliar products and take a trip someplace warm. Someplace where you don’t have to see the soybeans for a couple weeks. It just might make everyone happier, including your friends. For others, keeping that money in the bank may be the best stress reliever right now.

The bottom line is small soybeans or late-planted soybeans including double crop that do not reach full canopy by flowering probably have lost some yield potential. Cooler temperatures also reduce the chances of soybeans reaching full canopy by flowering.

In hindsight, the best management practice would have been to plant in 7.5-inch rows instead of wider rows (and a lot of soybeans are narrow rows). The narrow rows would have improved the chances of getting complete canopy closure by flowering. Foliar fertilizers and fungicides will not make up the difference in temperatures, planting date or row spacing.