May 12, 2020


US soybeans face higher frost risk 



With a low air temperature forecast on Saturday in the mid-20s, soybeans in US Michigan are more at risk of severe injury than corn, reported Monroe News.


Although 28 degrees F is a killing frost temperature, air temperatures are taken five feet above ground.


Soil temperatures do not mirror air temperatures, though will drop possibly into the 30s. The low soil temperature has already dropped to 47.7 on Wednesday morning.


The main reason to talk about soil temperatures in May is if the forecast is unfavourable for planting or spraying. The good news is that this planting season has been the best in the past four years. Since April 1, the Michigan State University (MSU) Enviroweather station in Deerfield has recorded only 2.08 inches of rain, compared to 5.01 inches last year, 5.15 inches in 2018 and 5.46 inches in 2017. The growing degree day units are up: 234 GDD since April 1 as compared to 214 GDD last year and 219 GDD in 2018.


Corn already planted and emerged will be at a high risk of severe injury, especially if soils are dry. Wet soils hold temperatures better than dry soils, so the injury potential partly depends upon the rainfall. Dark-coloured soils also radiate better and so offer more protection from radiational cooling that can kill leaf areas that are exposed to the sky, such as on a clear night.


Good news is that the growing point (the apical meristem) is still in the ground so any frosted above-ground tissue (leaves) will slow down plant growth as they first recover and then will resume producing new leaves. Any purple-leaf syndrome will be temporary. Farmers should plan to spend time scouting fields next week to determine the extent of injury or death of plants.


Farmers still planting soybeans over 150,000 seeds per acre, in any row spacing or tillage system, need to seriously look at the data from MSU and other Midwest land-grant universities over the past 20-plus years. As long as seed spacing is relatively uniform, fewer plants will produce more pods, while more plants will have less pods. In Monroe County, the data goes back to 1994 in planting-rate studies at the Herb Smith farm. Taller soybean plants may look nice, but may not yield more than shorter plants.


The key is pods per node and more nodes per plant. With a hard freeze forecast, soybeans are more at risk of severe injury than corn, therefore scouting emerged stands in the next week to 10 days may help farmers decide about replanting. Soybeans are vulnerable to frost injury for only a day or so as they are coming through the soil. Planting into cool and wet soils affects germination and early-season growth more than a night of frost.


Wheat is approaching the critical Feekes' growth stage 6, which is stem elongation. Now is a great time to apply the second half of a spring topdressing of nitrogen. Wheat prices are relatively better than corn or soybeans, so now is an ideal time to evaluate fields for yield potential. A BASF/MSU project a couple of years ago had participating farmers apply a plant growth regulator, with added nitrogen at Feekes stage 5 and again at stage 7, both of which are worth considering in highly managed fields.