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Integrated Pest & Crop Management


William J. Wiebold
University of Missouri
Plant Science & Technology
(573) 882-0621

Weather Challenges Still Plague Missouri Farmers

William J. Wiebold
University of Missouri
(573) 882-0621

Published: September 23, 2019

flooded corn field

According to the September 4 Farm Service Agency report, wet and flooded fields prevented Missouri farmers from planting 750,000 acres of corn and 480,000 acres of soybean. These are 21% and 9% of the intended acres (March 29 information) of corn and soybean.

Although the majority of intended acres were planted, many acres were planted well past their optimum dates for yield. NASS/USDA data indicated that about 50% of the planted corn emerged after May 26. The date for 50% emergence of soybean was June 23. Yield success from the late planted acres is highly dependent on weather conditions during July and August for corn and August and September for soybean.

Two weather parameters that greatly affect yield are temperature and precipitation during grain-fill. Several temperature parameters are presented in Table 1. I used five locations to represent regions of grain crop production throughout Missouri. Night temperature influences yield because it affects the rate of respiration. Warmer temperatures mean faster respiration and potential loss of yield. Respiration is required for life, but warm night temperature respires away sugars that could be used to create yield. Consensus of what night temperatures are "good" or "bad" for yield is difficult to find. I used 65F as the upper limit of temperature that may limit respiration rate and 75F for the lower limit of temperature that may stimulate rapid respiration. In other words, night temperatures below 65F are good for yield and above 75F are bad for yield.

Table1 Temperature parameters for four months at five locations within Missouri in 2019.

  Number of days with min. temperature less than 65F Number of days with min. temperature greater than 75F Average maximum temperature (F)
Jun Jul Aug Sep Jun Jul Aug Sep Jun Jul Aug Sep*
Corning (NW) 14 7 8 7 5 3 1 0 84.5 87.9 84.2 85.8
Novelty (NE) 24 8 18 10 0 0 0 0 80.4 86.3 80.5 81.3
Columbia (C) 20 8 17 9 0 0 0 0 80.3 85.3 82.6 83.2
Lamar (SW) 13 5 4 4 0 1 2 0 82.6 86.6 84.7 89.7
Portageville (SE) 6 3 2 1 0 8 2 0 85.8 89.2 88.7 90.8

* First 15 days of September, only. Source: Missouri mesonet

June nights were unusually cool throughout much of Missouri. At Novelty and Columbia, temperatures dropped below 65F more than two thirds of the nights. Cool nights decreased growing degree day accumulation and slowed corn development in June. Late planted corn would have benefited from warmer than normal daily minimum temperatures in June.

NASS/USDA reported that 80% of Missouri's corn crop had reaching silk stage of development by August 1 (about 2 weeks later than normal). This year, the majority of corn grain-fill occurred during August. Night temperatures were conducive to yield production with many nights possessing minimum temperatures below 65F and few, if any nights, where temperature remained above 75F.

Minimum temperatures during August were conducive to soybean yield production, also. Because of late planting, more than 50% of Missouri's soybean crop flowered after August 1. The most critical stage for soybean yield response to weather is R4. NASS/USDA uses the term "setting pods" which roughly corresponds to R3 or R4. The majority of Missouri soybean fields did not reach that stage until late August. This means that September weather is more important than normal to soybean yield in 2019. September night temperatures have not been a concern. But, recently, maximum temperatures have been 10 to 20 degrees above normal. These hot temperatures have had little direct effect on soybean growth, but they have increased water evaporation and that aggravates lack of rain.

Precipitation, either too much or not enough, has challenged most Missouri corn and soybean farmers during the entire growing season. Table 2 presents three precipitation parameters that may affect grain yield. In order to produce normal soybean and corn yields, a field should receive a little more than one inch of rain each week. I calculated the number of 7-day periods in June, July, August, and the first two weeks of September in which rainfall was more than one inch. If more than half of the days within a month (15 days or more) are part of a 7-day period with more than an inch, then the month is reasonably stress free. Because too much rain is also harmful, I calculated the number of 7-day periods with more than 3 inches of rain. Total rainfalls for the months are also presented.

Table 2 Precipitation parameters for four months at five locations within Missouri in 2019.

  Number of 7-day periods with more than 1 inch of precipitation* Number of 7-day periods with greater than 3 inches of precipitation* Total precipitation (inches)**
  Jun Jul Aug Sep Jun Jul Aug Sep Jun Jul Aug Sep
Corning 10 7 12 3 0 0 3 0 3.1 2.0 5.4 1.1
Novelty 18 7 11 0 5 0 0 0 7.4 2.7 3.2 0.7
Columbia 19 16 17 0 2 0 2 0 6.1 3.9 4.3 0.8
Lamar 17 18 21 0 4 5 1 0 6.2 5.9 7.3 0.4
Portageville 15 8 10 0 7 0 7 0 7.8 5.1 6.8 0.0

* Each day of the month begins a 7-day period, so there are 30 7-day periods in June and 31 7-day periods in July and August. 7-day periods for late June, July and August extend into the next month; for example, the 7-day period for August 29 extends from August 29 through September 4. There are 8 7-day periods between September 1 and 15.

** First 15 days of September, only. Source: Missouri mesonet

During summer 2019 dry weather has spread from north to south within Missouri. June was stressful in the Corning region and July in Novelty. Overly wet weather extended into August in SE and SW Missouri. Unfortunately, some fields that had been replanted in southern Missouri did not produce adequate stands because of heavy precipitation. The first two weeks of September have been too dry for late planted soybean and ultra-late planted corn.

One of the worse weather patterns for grain crop yield is wet weather in spring followed by drought stress during grain-fill. Wet fields during planting result in soil compaction increased disease potential. Often, root systems do not recover. Peak demand for water occurs during grain-fill. Smaller than normal and/or unhealthy root systems cannot supply adequate water to plants. The northern part of the state experienced this weather pattern with dry weather occurring as early as mid-June or July. Corn and soybean yields will be decreased as a result.

Because of the wet spring and normal double-cropping, soybean was planted late throughout the state. Many soybean fields are in the midst of rapid grain-filling as this article is written. Unfortunately, weather has turned dry with little or no rain after mid-August. A week of hot dry weather can negate "good" weather that occurred earlier. Although rain is predicted this weekend it will occur too late for most fields.

Hot dry weather during grain-fill can result in small seeds and even premature plant death. Corn grain test weight are often less than normal because of the stress. Seeds on soybean plants that die because of drought may remain green, and green color is often docked at points of sale. Farmers needed an unusually cool and wet August and September. Some parts of the state experienced that type of weather in early to mid-August, and I was optimistic. But, September has been too dry and hot to maintain grain-fill for late planted soybean fields and yield will likely be hurt.

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REVISED: September 23, 2019