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Irrigated cotton growers are questioning why, in some years, high yield potential cotton produces yields 20 to 30% below expectations. Ongoing research implicates both higher than long-term average day and/or night temperatures. High temperatures can decrease photosynthesis and increase respiration, leading to reduced seed production, reduced lint development, and unexpectedly lower yields. While cotton typically maintains canopy temperatures lower than air temperatures, high humidity, coupled with high air temperature, could lead to canopy temperatures above the optimum.
Georgia farmers with irrigated cotton typically expect 3-bale cotton yields. When their 2016 crop yielded 1100 to 1200 pounds per acre they were disappointed and surprised. John Snider, cotton physiologist, University of Georgia, says sustained high temperatures may have contributed to the lower than expected yields.
"If we have high day time temperatures that are reducing photosynthesis and high night time temperatures that are increasing respiration, that can reduce productivity," he says. "When we have these high temperatures, I've seen lower seed numbers, indicating reduced fertilization efficiency. Plants may produce the same number of bolls, but fewer seeds per boll result in smaller bolls and lower lint production. When heat stress is combined with insect and disease issues, the effects on yield are likely more pronounced than any given stress alone." (Snider, J., personal conversation, October 17, 2016)
An analysis of temperature extremes from June 1 to September 1, 2016 (a year when numerous Georgia cotton growers harvested lower than expected yields), compared to the 5-year average temperatures, may indicate that sustained higher than normal temperatures can negatively affect cotton yields. Figure 1 shows the number of days during the 2016 growing season that temperatures exceeded the 5-year average. Figure 2 indicates June 1 to September 1 seasonal high, low, and average temperature trends compared to the 5-year average.
Some growers also experienced reduced yield even when seed production was normal. Snider stated, "If high temperatures cause cotton to accumulate the same heat units over a shorter period of time during fiber development, this can result in reduced fiber quality and yield. If plants have normal seed and boll numbers and still experience lower yields, there could have been an effect of high temperature on fiber development."
Scientists at the University of Arkansas exposed cotton plants to normal (89.6 oF day time temperatures and 75.2 oF night time temperatures) and normal day time temperature but increased (86 oF night time temperatures) for 7 days following pinhead square stage (approximately 4 weeks after planting).1 Although the cotton was not taken to yield, the growth chamber studies showed significant effects on crop productivity brought on by the higher night time temperatures. Plants grown with higher night time temperatures had 23% and 36% lower photosynthesis rates compared to the control, 1 and 7 days after treatment initiation, respectively. Respiration in the heat-stressed plants increased 54% compared to the control on the first night. Seven days into the trial, heat-stressed plants had a 68% higher respiration rate compared to the control.1 Further analysis of the heat-stressed plants showed reduced carbohydrate content in the flower buds and a decrease of almost 50% in the number of flower buds per plant, thus reducing the plant's ability to reach optimum yield potential.
Heat stress on cotton, caused when cotton canopy temperatures rise above 82.4 oF, can reduce overal lint yields, delay crop maturity, and reduce lint quality. Air temperatures are typically significantly higher than canopy temperatures. Optimal performance occurs at canopy temperatures below 82.4 oF and serious yield losses can result once canopy temperatures exceed 86 oF. When canopy temperatures exceed 86 oF, flowers produce little or no pollen which leads to abortion of young 3-5 day old bolls. Young squares are also damaged, leading to low fruit retention. These damaged squares typically do not abort, but develop into smaller flowers that do not fully open, produce sterile anthers, and have what appears to be an elongated stigma. This is actually an illusion that results because the filaments supporting anthers fail to grow and elongate properly while the stigma grows more or less normally.2
Cotton growers, particularly those growing cotton without irrigation, may be able to hedge against reduced yield potential caused by high temperatures by selecting varieties that are identified as being more heat tolerant, although information on heat tolerance of available cotton varieties is limited and is an area where additional research is needed. Heat and drought stress can occur simultaneously, even in irrigated fields when ambient temperatures are high and crop water use rates exceed irrigation system capabilities. It may also be possible to avoid heat stress in some fields by stretching out planting dates or by planting earlier maturing varieties that may pollinate before high temperatues occur. Agronomist point out; however, that predicting high temperature extremes prior to planting is nearly impossible. They generally recommend selecting varieties with high yield potential with good insect and disease resistance and planting within the time window established for a given region for normal cotton production.