Harvest aids in indeterminate and determinate soybeans – application timing and value

Linda Benedict, Boudreaux, James E., Griffin, James L.  |  4/23/2008 3:49:42 AM

Soybeans in the foreground and background are the same variety planted at the same time. Soybeans in the foreground received a Gramoxone Inteon harvest aid application at R6.5 (50 percent seed moisture) and are ready to harvest. (Photo by Joseph M. Boudreaux)

Table 1. Asgrow 4403RR Group IV indeterminate soybean variety response to application timing of harvest aid versus a nontreated control (NTC) in 2006 and 2007.

Table 2. Asgrow 5903RR Group V determinate soybean variety response to application timing of harvest aid versus a nontreated control (NTC) in 2006 and 2007.

Joseph M. Boudreaux and James L. Griffin

Farmers have traditionally used harvest aids to desiccate weeds and improve crop quality and harvest efficiency. In recent years, harvest aids have become especially important in producing earlymaturing soybeans in Louisiana. Excessive foreign material and moisture associated with green plants at harvest can reduce the price producers receive. Additionally, as soybean plants remain in the field drying down, adverse weather conditions can reduce both yield and seed quality – and income to the grower.

Maturity Group IV (indeterminate) and Group V (determinate) soybean varieties are widely grown in Louisiana. These varieties differ in physiology and vegetative growth that occurs after the plants begin to flower.

Indeterminate soybeans begin flowering at the lower nodes of the plant, and flowering continues upward toward the top. The plants continue to grow for several weeks after flowering begins. Seeds on the bottom of the plant will mature first, and the most immature seeds will be in top of the plant.

Determinate soybeans, by contrast, begin flowering in middle of the plant, and flowering proceeds both upward and downward. For the most part, these plants quit growing when flowering begins. The most immature seeds will be on both the top and bottom of the plant, but most seeds mature at the same time.

Because indeterminate varieties continue to grow taller after flowering begins, plants tend to retain leaves longer. For this reason, a desiccant may be of greater value with these varieties.

Gramoxone Inteon and sodium chlorate are labeled as harvest aids/desiccants for soybeans. The label for Gramoxone Inteon states that for indeterminate varieties it should be applied when at least 65 percent of the seed pods have reached a mature brown color or when seed moisture is 30 percent or less. For determinate varieties, Gramoxone Inteon should be applied when plants are mature – when beans are fully developed, half of the leaves have dropped and remaining leaves are yellowing. The timing of application based on the label is unclear and may be difficult for a grower or consultant to interpret.

Research conducted in 2006 and 2007 evaluated timing the application of the harvest aids Gramoxone Inteon at 1 pint per acre, Gramoxone Inteon at 1 pint plus Aim at 1.4 ounces per acre, and sodium chlorate at 4 quarts per acre. Aim was applied with Gramoxone Inteon to represent situations when broadleaf weeds were present. Soybeans were treated when average seed moisture was at 60, 50, 40, 30 or 20 percent, which was determined by collecting seeds from the top four nodes of plants where seeds would be the least mature. Seeds were weighed, dried and re-weighed to calculate moisture. In general, seeds lost 10 percent moisture in seven to 14 days, depending on the year.

Experiments were conducted using a maturity Group IV indeterminate variety and a Group V determinate variety. Soybeans were harvested and yield was determined when seed moisture was around 13 percent. The number of days soybeans were harvested before the nontreated control was recorded to further quantify the value of the harvest aid treatments. Harvested soybean samples were not graded at the elevator, and economics of the treatments were not determined. Further research, however, is under way to determine if improvement in grade and seed quality justify harvest aid use.

All harvest aid treatments were equally effective in desiccating soybean foliage for both varieties, but application timing had a significant effect. The Group IV indeterminate variety showed no statistically significant difference in yield among the harvest aid application timings in 2006, but differences were observed in 2007 (Table 1). Yields were reduced by an average of 10.3 bushels per acre (17 percent) when the harvest aid treatments of Gramoxone Inteon and sodium chlorate were applied at 60 percent average seed moisture compared with the application at 50 percent moisture. The yield at 50 percent seed moisture, however, was equal to the other treatments. When harvest aid was applied at 40 percent and 50 percent moisture, soybeans were harvested nine and 14 days earlier than the nontreated soybeans in 2006 and five and 10 days earlier in 2007.

For the Group V determinate variety when harvest aid was applied at 40 percent average seed moisture, yield was equal to the nontreated control in both years (Table 2). In 2006, yield was reduced by 6.2 bushels per acre (12.1%) with application at 60 percent moisture compared with 40 percent moisture. Yield was equal when harvest aid was applied at 40 percent and 50 percent seed moisture. In 2007, yield was reduced by 17.7 bushels per acre (27.7%) with application at 60 percent moisture and reduced by 13 bushels per acre (20.3%) at 50 percent moisture when compared with 40 percent seed moisture. Yield reductions were attributed to reduced seed weight, which would be expected when soybean leaves are removed before seed fill is complete. When harvest aid was applied at 40 percent moisture, soybeans were harvested seven days earlier than the nontreated soybeans in 2006 and 15 days earlier in 2007.

Application timing was more flexible with the indeterminate variety because the most immature seeds are present only in the top of the plant instead of in both the top and bottom of the plant as with the determinate variety. Research results show that harvest aid – either Gramoxone Inteon or sodium chlorate – can be of value by desiccating leaves and stems and speeding up harvest of both indeterminate and determinate soybeans. A harvest aid may be especially valuable for indeterminate soybeans because terminal buds continue to grow several weeks after flowering and plants retain leaves longer. For soybean varieties grown in Louisiana, harvest aids can be applied when seeds from the uppermost four nodes of the plants are around 50 percent moisture (at R6.5) without negatively affecting yield.

Procedure to determine when to apply a harvest aid to indeterminate and determinate soybeans
  • Collect pods from the uppermost four nodes of plants at random across the field.
  • Open pods and look for separation of beans from the white membrane inside the pod.
Beans should be easy to shell from the pods. If this is observed for all pods collected, then seed are at physiological maturity (around 50 percent moisture) and have reached maximum dry weight. Many leaves on plants have dropped by this time, and the remaining leaves are yellow. It is safe now to remove the remaining leaves chemically without affecting seed weight. Another sure-fire way to know that it is safe to apply a harvest aid is when one normal pod on a main stem has reached mature color. This growth stage is R7, and harvest should occur in about three weeks. If after opening the pods not all beans easily separate from the pod wall, application of harvest aid will result in some yield loss because of lower seed weight, and the effect will be greater for determinate varieties. Growers will need to decide if the yield loss can be offset by earlier harvest or improvement in grade. A reduction in price received per bushel because of dockage associated with foreign material may more than offset the cost of a harvest aid application.

(This article was published in the winter 2008 issue of Louisiana Agriculture.)
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