Cotton topping must be done at the right time and promptly; seizing the moment is key
Cotton has an indeterminate growth habit. Under certain temperature conditions, the terminal bud keeps growing and extending, and continuously differentiates new stems, leaves and fruiting branches. Topping is the manual removal of the terminal bud so that it no longer produces new fruiting branches, allowing the vast majority of bolls to open before frost. Therefore, this is a key operation for artificially regulating the cotton growth period. Before topping, the cotton plant has apical dominance, with water and nutrients supplied preferentially to the terminal bud; after topping, they are supplied preferentially to the organs growing and developing fastest. It can thus be seen that topping affects the transport and distribution of nutrients in the cotton plant, so it is also an operation with a profound effect on the physiological activities of cotton. Since the main stem no longer produces new leaves after topping, it can also be regarded as artificially terminating vegetative growth and turning it entirely toward reproductive growth.
When is the most suitable time to top cotton? As the saying goes: "Topping by branches, not by time; topping by time, not by branches." This means that you should not calculate only by date, nor only by the number of fruiting branches, but should comprehensively consider two factors: the season and the growth condition of the cotton plant. In the central-southern Hebei cotton region, it is generally more suitable to top after 14-15 fruiting branches have formed, around July 15-20.
How many fruiting branches should be retained per plant for the greatest suitability? This depends on density, soil fertility, cotton growth vigor, boll setting in the middle and lower parts, and variety characteristics: (1) With high density, the requirement for boll setting per plant is lower, so appropriately fewer fruiting branches should be retained, while with low density, appropriately more should be retained; (2) On fertile land with vigorous cotton growth, appropriately more fruiting branches should be retained, while on thin, poor land with weak growth, fewer should be retained; (3) If there are many bolls in the middle and lower parts, retain fewer fruiting branches in the upper part, and if there are few bolls, retain more; (4) For early-developing varieties, retain fewer fruiting branches in the upper part, and for late-developing varieties, retain appropriately more; (5) Hybrid cotton has a greater potential for boll setting per plant, so appropriately more fruiting branches should be retained than for conventional cotton.
In specific operation, if by mid-July early-developing cotton plants have reached the required number of fruiting branches, topping can begin, and it should be finished by July 20; for high-yield fields and late-developing fields, it can be delayed a few more days. If, due to disasters or other special circumstances, there are too few fruiting branches or bolls, the topping time can be postponed to the end of the month, and after topping, spray gibberellin and foliar fertilizer to stimulate flower bud formation (see Member Newsletter Issue 92 for details). As for the topping method, removing only the small tip is best, as it both reduces nutrient consumption by the cotton plant and promotes earlier transformation. After topping, strict precautions must be taken against pests, and buds and flowers must no longer be allowed to suffer damage. The value of cotton topping lies in doing it at the right time and promptly; under no circumstances should the timing be missed for fear of shedding. Some cotton fields fail to achieve a boll cover on top in autumn, and late topping is one important reason.