Timing and Quality Managment

Proper timing of defoliation is key to maintaining fiber quality at the end of the season. Poor defoliation timing can be economically costly. Defoliating too early lowers lint yield and fiber quality, especially micronaire. Defoliating too late increases the likelihood of boll rot and lint damaged or lost due to weathering. Defoliating too late will also lead to discounts from high micronaire values with certain varieties as well as increase the possibility that defoliant activity may be inhibited by lower temperatures.

Percent Open

It is generally safe to defoliate when about 60 percent of the bolls are open. But this strategy may not work well in situations where the crop is set faster or slower than normal. A crop set over a long period may have a fruiting “gap” due to fruit loss associated with stress or insect pressure at peak bloom. This type of crop may have a high proportion of immature bolls at 60 percent open. Defoliation at 60 percent open would cut short the development of the top bolls and reduce yield and micronaire. On the other hand, a crop set in a short period of time, such as three weeks, could possibly be safely defoliated prior to 60 percent open boll.

Nodes above Cracked Boll

Another method that is often used to time defoliation is counting the nodes above cracked boll (NACB). This is done by selecting plants with a first-position cracked boll (cracked enough that lint is visible) and counting the nodes above the cracked boll up to the highest node that has a harvestable boll. This technique places more emphasis on the unopened portion of the crop than the percent open. An NACB of 4 is usually safe for defoliation. If you have low plant populations (less than two plants per foot of row), an NACB count of 3 would be safer. Low plant population results in a less mature crop because of the number of bolls set on vegetative branches and outer positions of the fruiting branches.

Heat Unit Accumulation

Measuring accumulated heat units (DD 60's)past physiological cutout or nodes above white flower=5 (NAWF=5) is another way to help schedule defoliation applications. In general, cotton that has accumulated 850-950 heat units or DD60's past cutout (NAWF=5) is safe to defoliate.

Cut Boll Technique No matter which technique is used, producers should also cut and examine unopened bolls to ensure that harvestable bolls are mature. Bolls need 40 to 60 days to mature, depending on temperature. In cool weather, bolls will need extra time to mature. A boll that is set early in the bloom period will often mature in about 40 to 45 days, whereas a boll set later in the bloom period may require about 50 to 60 days to mature. The last effective bloom date varies for different parts of the cotton belt, but would be between August 15 and September 15 for most parts of the United States cotton belt. For example, in North Carolina, bolls set (white blooms) after August 15-25 are likely to never mature. Producers should walk each field and decide which bolls they intend to harvest and examine these bolls to determine whether they are mature. The younger bolls in question will be the bolls toward the top and outer portions of the plant.

Sliced bolls to determine boll maturity.

Bolls will be mature enough for defoliation when:

1. They are hard and difficult to slice into cross sections with a sharp knife. The fibers should string out when the boll is cut. If the fibers do not string out, the boll is not mature.

2. The seed coat is light brown and the kernel completely fills the seed cavity with no jelly in the center. The seed coat is a pearly white in young bolls and turns from white to black as the boll matures. When the seed coat becomes light brown, the boll is mature enough not to be adversely affected by harvest-aid chemicals.

Points to Ponder

Defoliants work best on mature cotton under warm, humid, sunny conditions. Cool temperatures at the time of application and for the 3 to 5 days afterward can retard the activity of defoliants and cause less than desirable defoliation. If possible, defoliants should not be applied during cool snaps. Better defoliation will occur if you can wait for a warm spell that is predicted to last for at least 3 to 4 days.

Defoliation is not always justified. Cotton that is completely cutout with “tough” leaves may not need defoliation if harvested with care. In this situation, it is important not to pick too early or late in the day to avoid excess moisture. If you are considering picking without defoliation, pick a trailer full and see how well it cleans up at the gin. If the gin can clean the lint so that it will grade a 41 or better, then defoliation may not be needed. Harvesting without defoliation is much rarer now than it was in the recent past due to the abundance of cotton stored in modules and the recent shift in the desirability of achieving better grades than 41 for the export market.

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