Monitoring cotton growth can help growers with management decisions, like plant growth regulator (PGR) applications, to help increase cotton fiber quality and yield potential.
Cotton product growth habit and response to PGRs is important for developing a growth management strategy.
Cotton is a perennial plant with varying levels of indeterminacy that are independent of a maturity group. Understanding the degree of indeterminacy can help with placement and management of a cotton variety.
Varieties with a more indeterminate growth habit can sustain terminal growth longer than less indeterminate varieties. Although node counts may be similar prior to bloom, more indeterminate varieties can continue to produce nodes while less indeterminate varieties will move toward cutout. More indeterminate varieties can produce nodes and fruit even in stressful growing conditions, but maturity can be delayed for late-set bolls. Cotton specialists recommend to monitor cotton plant growth and apply plant growth regulators (PGRs) to control excessive growth, especially for more indeterminate varieties.
Several measurements can be used to help monitor cotton growth (Table 1). Using a combination of these measurements can provide a more accurate evaluation of crop growth. During bloom, growers and consultants should record height and mainstem nodes, nodes above white flower (NAWF) counts, square retention, and maximum internode distance on a weekly basis.
Plant height. Mainstem height is one of the most common cotton measurements and is useful in making management decisions when combined with other information. Generally at first bloom or shortly thereafter, cotton is growing at its maximum rate. A growth rate of about 1 inch per day is the upper limit of acceptable growth.
Mainstem nodes. Development of mainstem nodes is also close to its maximum rate near first bloom. There should be a new mainstem node about once every 2.5 to 3 days, or approximately one every 40 to 50 DD60s. Water availability and carbohydrate production are the main influencers of mainstem node development.
Squaring nodes. The maximum number of fruiting branches prior to bloom, or the maximum NAWF, is an excellent indication of vigor. Measurement of NAWF is effective because it directly corresponds to energy absorption by the reproductive and vegetative demands of the plant. This measurement monitors the difference between the rate at which squares reach bloom and the rate that new mainstem nodes are produced.
As NAWF declines, nodes are being produced slower than first fruiting position squares reach bloom. This value ranges from 5 to 10, with 8 and 10 nodes being most common if stress has not been a significant factor. Values of 7 or less at first bloom indicate low vigor and using a PGR is not recommended until the stress is alleviated. If the value is closer to 10 and/or does not decline at a rate of about 1 node per week, vigor is high and using a PGR may be beneficial. NAWF of 5 indicates the plant is entering cut-out.
Maximum internode distance (MID). The distance between the 4th and 5th nodes from the terminal is the most sensitive measurement of current vigor. It encompasses the effects current inputs are having on growth and the relationship between carbohydrate supply and demand. After first bloom, if this distance exceeds approximately 3 inches, current growth is vigorous and needs control. If it is less than 2 inches, growth is limited and may need investigation. In some situations, cotton with a MID under 2 inches may be a normal response to retention, PGRs, and environment. Mepiquat chloride rate and timing sticks (Figure 1) are available to provide recommendations according to the average internode length of the top five nodes of the plant. The longer the internode length, the higher the recommended rate.
Mepiquat chloride and mepiquat pentaborate both contain mepiquat which is an anti-gibberellin growth retardant that reduces plant cell enlargement to help balance vegetative and reproductive growth. When applied to cotton, it can help control rank growth by reducing stem elongation at newly formed internodes. The application of PGRs can help increase fruit retention and promote earlier maturity, reducing the crop’s risk of late-season insect damage, boll rots, and harvest losses. Mepiquat applications have been linked to increased cotton yield potential when applied at the optimum rate and timing for the variety and field planted.
Understanding the growth habit of a variety is important to manage vegetative growth. Certain varieties can have a more aggressive growth pattern and PGR rates and timing must be managed accordingly. The projected response of Deltapine® cotton varieties to the application of mepiquat is provided in Table 2. Varieties that are most responsive may need lower rates or fewer applications of mepiquat, whereas varieties that are least responsive may require timely early-season applications at higher rates or multiple applications to control vegetative growth.
For cotton varieties in the moderately and least responsive categories, growers should plan to apply mepiquat at 8 to 10 nodes (match-head square) and aggressively apply mepiquat if there is a history of rank growth, if the crop received early rainfall, or was planted behind a crop where a high nitrogen rate was applied. Mismanagement of PGRs can have a negative effect on yield potential, especially when applied too early, when cotton is stressed, or when applied to a less indeterminant cotton variety.
In-season cotton plant monitoring provides the best information to determine application rate and timing of growth control measures. Every cotton field is under different conditions so growth management strategies should be tailored to each field situation.