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Several cotton plant measurements can be used to ensure that growth and development are occurring normally. A combination of these measurements may give a more accurate evaluation of crop growth. During bloom, farmers and consultants should continue to record height and mainstem nodes, nodes above white flower (NAWF) counts, square retention, and maximum internode distance on a weekly basis.
Height of the mainstem is one of the most common cotton measurements. It can be 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. Growth more rapid than this indicates an alteration in management is needed and using a plant growth regulator (PGR) may be beneficial.
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. More important than the number of mainstem nodes on a given day is whether or not the number of days or DD60s between new nodes is increasing. If this is not occurring, or vegetative growth is not slowing, by the third or fourth week after bloom, an investigation is warranted.
The maximum number of fruiting branches prior to bloom, or the maximum NAWF, is an excellent indication of vigor. Measurement of NAWFis 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 new vegetation and 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 when 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 as noted by NAWF values increasing to normal range. 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.
The maximum internode distance (MID - the distance between the 4th and 5th nodes from the terminal) is the most sensitive available measure of current vigor. It encompasses all effects current inputs are having on growth. It also reflects the current 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 having a MID under 2 inches may be a perfectly normal response to retention, PGRs, and environment.
As cotton bolls begin to grow, fiber cells on the seed coat begin to elongate. These cells continue elongation for 3 weeks, starting with the day of flowering. Stress during this time will reduce fiber length. The following 3 weeks, a layer of cellulose is deposited daily on the inside of the cotton fiber. This is called secondary wall thickening and directly corresponds to fiber strength and micronaire. Stress during this second phase of fiber development can have detrimental effects on fiber quality.
Micronaire is a measure of fiber weight per unit length and the reading is used as an indicator of fiber fineness which also relates to the maturity of the fiber. Fiber fineness and diameter may be greatly impacted by environmental conditions, but it also has a significant genetic component. If fiber development is terminated prematurely, finer fibers will result in low micronaire. Conversely if environmental conditions are good, fibers will continue to mature, resulting in coarse fibers with high micronaire.
Micronaire can vary due to boll maturity by field, but also by the location of the boll on the plant. Bottom bolls have relatively higher micronaire and upper, less mature bolls have lower micronaire. Fiber from the bolls is blended at harvest to establish micronaire for the field/variety. For this reason, harvest termination and in-season crop management, along with localized weather conditions can influence crop micronaire measurements.
Remember, higher yielding crops are generally higher micronaire crops, while crops terminated prematurely either by weather or harvest aids tend to have lower micronaire.
Sources: Ritchie, G.L. et.al. 2007 Cotton growth and development. University of Georgia Cooperative Extension. Bulletin 1253. http://www.caes.uga.edu (verified 06/25/2014); 2014 NC Cotton Information. http://www.ces.ncsu.edu (verified 06/25/2014).