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Low Temperature Damage To Corn And Soybean

By Jonathan Kleinjan
 
Row-crop planting in South Dakota began relatively early in the spring of 2015 and has progressed at a rapid pace. As of May 17th, 46% of corn was emerged, well ahead of the five-year average of 21%. Soybean emergence was recorded at 4%, only slightly higher than the five-year average of 3%. Early planting may increase yield potential in some years but also puts crops at risk for damage caused by early-season low temperatures. 
 
 
Early planting may put crops at risk for damage caused by early-season low temperatures.
 
Temperatures of 32°F and lower were observed in the central and northeast portions of the state during the early morning hours of May 19th, 2015. Producers in these areas of the state may want to evaluate individual fields for crop damage, as some locations (especially low-lying areas) may have actually seen lower temperatures than those recorded by the weather stations.
 
Frost & Freeze: Effects on corn & soybeans
 
What is the difference between a frost and a freeze? A frost event occurs when the temperature of the soil surface is 32°F or lower while a freeze occurs when the air temperature is 32°F or lower at five feet above ground level. It is important to remember that, if atmospheric conditions are right, it is possible for a frost to occur even when air temperatures are slightly higher than freezing. While a mild frost should cause minimal damage to young crops, a hard freeze (28°F or colder for 2 hours) will most likely be lethal.
 
The growing point for corn plants remain below the soil surface until the V6 growth stage (the plant is approximately 1’ in height) and is generally considered safe from freezing air temperatures. However, if air temperatures remain below 28°F for more than a few hours, the growing point may be injured or killed regardless.
 
Soybeans are more likely to be damaged by low temperatures due to the fact that the growing point is above ground as soon as the crop emerges. The soybean plant develops axillary buds at each leaf node, including the cotyledons. If any of these buds survive a low temperature event, recovery is possible. However, if the plant is damaged to a point below the cotyledons, seedling death will follow.
 
Assessing plant stands after low temperatures
 
Immediately following a frost or freeze, leaves on young corn and soybean plants may initially blacken and then become necrotic (wither and turn brown). This is not the time to make decisions about replanting. The actual extent of damage should be assessed 3-5 days after the event or when a few days of favorable growing conditions have returned. Corn plants that will survive should either have new leaf tissue emerging from the whorl or a growing point that is white/light yellow and firm. The growing point can be observed by splitting the seedling lengthwise and examining the area ½ to ¾” below the soil surface. A decayed and discolored growing point indicates a dead plant. Soybean plants should have new leaf tissue emerging from one or more of the undamaged nodes. Plants that still look dead at this point will probably not recover.
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