Why are there so many squares and bolls laying on the ground?
Boll retention is a complex interaction of hormones, nutrients, and the environment. Plant growth and development can be limited by the supply of carbohydrates available to the plant. Stressed plants adapt to limited resources by shedding squares (Figure 1) and young bolls to retain larger fruit on the plant.1 The squares and bolls may not shed until several days after the stress occurred.2
Soil moisture. If water is not available to the plant, squares and small bolls can shed until moisture improves. Saturated soils can also lead to shed if nutrients have leached through the soil or if roots are unable to get enough oxygen.1
Poor light penetration. Cotton plants rely heavily on the supply of carbohydrates from subtending leaves (the leaves that are located closest to each boll). Bolls that are set lower on the plant need light to penetrate through the canopy to the subtending leaves for carbohydrate production.3 Rank growth or a high plant population may prevent light penetration and lead to square and boll shed. Cloudy weather can also limit light penetration and the plant may shed small bolls and squares. If cloudy weather arrives with rain, pollination can be reduced and poorly pollinated flowers can also shed.1
Very high temperatures. Cotton is a tropical plant and tolerates heat well, but extreme heat can lead to plants using carbohydrates quickly to keep cool. This is especially a problem when nighttime temperatures are high because the plants never get a break and must constantly respire to keep cool. Young bolls and squares will shed so the plant can maintain the larger set bolls.1 Lower temperatures can slow boll development and may cause shedding.3
Insects and diseases. Shedding from insects and diseases can be direct or indirect. Direct feeding on squares or bolls can lead to shed or provide entry for diseases that can result in shed. Cotton fleahopper, Lygus spp. (eg. tarnished plant bug), cotton bollworm, pink bollworm, and tobacco budworm are known to feed on squares and bolls.1 Indirect damage can result from reduced leaf area (leaf feeding and lesions) or damage to the vascular system.1,2
Nutrient availability. The overapplication of nitrogen (N) can lead to rank growth, but it is more common for N to be a limiting factor. N is a critical component for plant development and is directly linked to flowering frequency and duration.1 Potassium promotes overall leaf health, which can help plants maintain a heavy boll load.4 A complete, carefully planned nutrient package, including calcium, boron, and zinc, is needed to reach optimal growth and development.1
Time of season. Shed increases as the season progresses. This is especially the case for crops with a heavy boll load.1 Toward the end of the season, the roots have maxed out what nutrients they can take up and plant resources are used for boll maturity. Shed is not typically a concern at this stage.
When should I worry about boll shed?
If the crop has shed a lot of bolls before a good crop of bolls have set, it is important to figure out why the plants are not setting bolls and manage the crop in a way that promotes flowering and boll production.
Can boll shedding be prevented?
Yes and no. Some level of shedding throughout the season should be expected. Shedding is the plant’s response to stress, which is inevitable even in ideal growing seasons. Controlling every potential stress during the season is impossible but mitigating what you can helps preserve boll retention and yield potential.
Select plant populations based on the growing conditions and management of the acres to be planted. Help the crop establish a good root system early in the season to increase access to nutrients and moisture throughout the season. If irrigation is available, use it for crop development and cooling down the crop during extreme heat. Help control rank growth with timely plant growth regulator applications. Protect early-set and first-position bolls. Manage damaging insects and diseases. Apply adequate amounts of nutrients, including minor nutrients, and base fertility required from a soil test.1,2
Managing stress for a cotton crop is not unlike personal stress management. Understand that there are environmental conditions that are out of your control, make sure the crop has adequate nutrition, be assertive and organized when making management decisions, and keep notes of when shedding was noticeable. Was it after cloudy weather? Extreme heat? Was the pivot turned on during pollination? Understanding potential stresses can help when making management decisions for maintaining boll retention and yield potential.
1 Guinn, G. 1982. Cause of square and boll shedding in cotton. U.S. Department of Agriculture. Technical Bulletin No. 1672. http://cotton.tamu.edu/General%20Production/Causes%20of%20Square%20and%20Boll%20Shedding%20in%20Cotton-complete.pdf.
2 Hake, K., Guinn, G., and Oosterhuis, D. 1989. Environmental causes of shed. National Cotton Council. Physiology Today. https://www.cotton.org/tech/physiology/cpt/plantphysiology/upload/CPT-Dec89-REPOP.pdf.
3 Verbree, D. 2013. Cotton fruit-shedding – who’s to blame. University of Tennessee. http://news.utcrops.com/2013/08/cotton-fruit-shedding-whos-to-blame/.
4 Stewart, S. 2012. Fruit loss in cotton and potassium deficiency symptoms. The University of Tennessee. UTcrops News Blog. http://news.utcrops.com/2012/07/fruit-loss-in-cotton-and-potassium-deficency-symptoms/.
Web sources verified 08/27/19.