How to Grow Cotton to Prevent Square and Boll Shedding

(1) General patterns of square and boll shedding

① Ratio of square and boll shedding: In square and boll shedding, the overall trend in the ratio of square shedding to boll shedding is that the boll shedding rate is higher than the square shedding rate, with a ratio of about 3:2.

② Age of squares and bolls at shedding: After squaring, shedding is greatest at 11-20 days, while shedding of large squares over 20 days old is less. Young bolls 3-8 days after flowering are most prone to shedding, with 3-5 days being the most common, while young bolls over 10 days old rarely shed.

③ Position of square and boll shedding: Generally, there is less square and boll shedding on lower fruiting branches and more on upper fruiting branches; there is less shedding of squares and bolls near the main stem and more on those far from the main stem. However, when planting density is too high, fertilizer and water are excessive, and cotton plants grow excessively vegetatively, the positions of square and boll shedding are the opposite of the above.

④ Period of square and boll shedding: In the period of square and boll shedding, generally there is very little shedding before the early flowering stage of cotton, then it gradually increases, reaching a peak at the full flowering and boll-setting stage, and afterward gradually decreases again. According to research, the number of shed squares and bolls during the full flowering and boll-setting stage from late July to early August accounts for about 56% of the total number of shed squares and bolls.

⑤ Differences in square and boll shedding among different cotton species and varieties. According to statistics, upland cotton has the highest shedding rate, Asiatic cotton is next, and sea-island cotton is the lowest. Among upland cotton varieties, the difference in square and boll shedding rate is relatively large, with a difference of up to about 35%.

(2) Causes of square and boll shedding

① Physiological shedding. Physiological shedding is the basic cause of square and boll shedding, accounting for about 70% of the total shedding rate. There are many factors causing physiological shedding, mainly the following: First, insufficient organic nutrients or improper distribution. When external environmental conditions are unsuitable, cotton plants grow weakly or excessively vegetatively, causing insufficient organic nutrients or improper distribution within the cotton plant, so that squares and bolls cannot obtain sufficient organic nutrients and shed. Among external environmental conditions, the factors with the greatest impact on physiological shedding are fertilizer, water, light, and temperature. Second, lack of fertilization. Unfertilized young bolls, due to weak growth and metabolic intensity and poor nutrient absorption capacity, inevitably shed. There are many reasons affecting fertilization; adverse environmental conditions such as rainfall, high temperature, and drought during flowering will destroy pollen and the pollination and fertilization process. Third, imbalance of plant hormones. The cotton plant contains five major categories of endogenous hormones: auxin, gibberellin, cytokinin, abscisic acid, and ethylene. After the content of these hormone-like substances changes, the hormones lose their balanced state, causing square and boll shedding.

② Diseases and insect pests. Diseases and insect pests can directly or indirectly cause square and boll shedding. Insect pests include mirids, cotton bollworm, cotton aphid, and others; diseases mainly include Fusarium wilt, Verticillium wilt, and red-leaf stem blight.

③ Mechanical damage. During the cotton growing period, careless field operation and management, or attacks by hail, storms, and rain, will damage branches, leaves, squares, or bolls, causing square and boll shedding.

(3) Main measures to prevent square and boll shedding

① Improve fertilizer and water conditions. This is mainly aimed at poor and thin cotton fields. In such cotton fields, fertilizer and water supply is insufficient, and cotton plants are prone to premature senescence in the later stage. Irrigation and topdressing have a significant effect on reducing square and boll shedding.

② Regulate the relationship between vegetative growth and reproductive growth well. This is mainly aimed at cotton fields with better soil fertility. Such cotton fields are prone to excessive vegetative growth. Through comprehensive cultivation measures such as fertilizer and water management, intertillage, pruning, and the use of growth regulators, the relationship between vegetative growth and reproductive growth should be coordinated well, so that organic nutrients are distributed reasonably and square and boll shedding is reduced.

③ Use reasonable close planting to improve light conditions in cotton fields. Mainly by establishing a reasonable population structure, reducing shading, improving field light conditions, and increasing light energy utilization, thereby reducing square and boll shedding.

④ Strengthen integrated control of diseases and insect pests. Reduce square and boll shedding caused by diseases and insect pests to a minimum.