James L. Griffin and Joseph M. Boudreaux

Harvest aids are herbicides applied to the crop late in the growing season to dessicate leaves and accelerate plant drying. This can result in earlier harvest and improved crop harvest efficiency and seed quality. Because soybean seed fill is dependent on the presence of soybean leaves, timing of harvest aid application is critical. If applied too early before all of the seed on the soybean plant has sufficiently matured, seed weight and yield can be reduced.

The following study was conducted to determine the precise timing for application of harvest aid – in this case paraquat sold as Gramaxone Inteon – to soybeans in Louisiana. Soybeans grown in Louisiana are of two types, indeterminate and determinate. For indeterminate soybeans, flowering and seed set begin in the lower portion of the plant and proceed upward; terminal buds continue growing several weeks after flowering begins. There can be considerable difference in seed maturity on the plant with bottom seed reaching maturity first. With the variation in seed maturation, indeterminate soybeans tend to retain leaf material, and stems remain green later into the growing season. In contrast, for determinate soybeans flowering and seed set are initiated in the middle portion of the plant and proceed toward the top and bottom; terminal bud growth ceases when flowering begins. While there may be some slight difference in seed maturity on the plant, most seed mature at the same time.

The label for Gramoxone Inteon states that application to indeterminate soybean varieties should be made 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, paraquat should be applied when plants are mature, i.e., beans are fully developed, half of the leaves have dropped, and remaining leaves are yellowing. The label also states that soybeans should not be harvested earlier than 15 days after application. Where morningglory vines are present, 15 days may be needed to kill plants. For both soybean types the goal would be to apply Gramoxone Inteon as early as possible to expedite harvest but not too early before all seed on the soybean plant have reached maximum dry weight (physiological maturity), when yield loss can occur.

The Study
Indeterminate (Asgrow 4403RR) and determinate (Asgrow 5903RR and Asgrow 6202RR) soybeans were treated with Gramoxone Inteon at 1 pint per acre (maximum label rate). Applications were made when moisture content of seed collected from pods at the four uppermost nodes of plants averaged 60 percent, 50 percent, 40 percent, 30 percent and 20 percent (plus or minus 2 percent). To determine seed moisture, pods were hand shelled and seed were weighed, oven-dried and re-weighed to calculate average moisture percentage.

For the indeterminate variety, Gramoxone Inteon application at 60 percent average seed moisture reduced yield 10 bushels per acre compared with the nontreated (Figure 1A). At this timing, leaves had not begun to yellow and pods were green. At 50 percent average seed moisture, soybean yield was 60 bushels per acre and was not negatively affected compared with the nontreated. Soybeans were harvested seven and 10 days after application depending on year and around 14 days before the nontreated. When application of Gramoxone Inteon was delayed until 40 percent average seed moisture or less, yield was equal to the nontreated and soybeans were harvested around nine days earlier than the nontreated.

For the determinate variety, Asgrow 5903RR, Gramoxone Inteon application at 60 percent average seed moisture reduced yield 13 bushels per acre compared with the nontreated (Figure 1B). Application at 50 percent average seed moisture reduced yield 9 bushels per acre. At 40 percent average seed moisture or less, soybean yield was 59 to 61 bushels per acre and was equal to the nontreated. Soybeans treated at 40 percent seed moisture were harvested 12 and 14 days after application, depending on year, and around seven days before the nontreated. For the determinate variety, Asgrow 6202RR, Gramoxone Inteon application at 60 percent seed moisture reduced yield 11 bushels per acre compared with the nontreated (Figure 1C). When application was delayed until 40 percent average seed moisture or less, soybean yield was 57 to 58 bushels per acre and was equal to the nontreated Soybeans treated at 40 percent seed moisture were harvested seven days after application and around 12 days earlier than the nontreated. For both indeterminate and determinate soybean varieties, reductions in yield were accompanied by reduced seed weight.

Soybean yield was not negatively affected when Gramoxone Inteon was applied to the indeterminate variety at 50 percent average seed moisture and to the indeterminate varieties at 40 percent average seed moisture. The difference in response was attributed to where flowering and seed set is initiated on the plant and the effect on seed maturity. For the indeterminate variety the most immature seed would be present in the top of the plant. In contrast, for determinate soybean, the most immature seed would be present in both the top and bottom of the plant.

It is not practical that growers determine application timing for a harvest aid by collecting and drying seed to quantify moisture. Using percentage leaf drop or percentage of pods that have reached mature brown color is not the best indicator for harvest aid timing. Rather, harvest aid timing should be determined based on close observation of seed within the pods.

Procedure to Determine When to Apply Harvest Aid to Soybeans

• Begin looking closely at soybean plants when yellowing of leaves is first observed.

• Collect pods from the top four nodes of plants at random across the field.

• Open pods and look for separation of beans from the white membrane of the pod wall. Seed should be easy to shell from the pods.

• If separation of beans from the pod wall has occurred for all pods collected, then all seed on the plant have reached maximum dry weight. It is now safe to remove leaves without affecting seed weight.

• At this time leaves have begun to yellow, and some leaves may have dropped. This can vary greatly depending on variety and environmental conditions. Also, some of the pods on the lower portion of plants have begun to turn yellow.

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 reduced seed weight, and the effect will be greater for determinate than for indeterminate varieties.

In recent years, there has been an increased incidence of the "green plant malady" where soybean plants retain leaves, and stems remain green after soybean seed have reached harvest maturity. Application of Gramoxone Inteon has decreased green leaf retention, reduced percentage green stems and seed moisture of harvested soybean, and promoted earlier harvest.

James L. Griffin, Lee Mason LSU Alumni Association Professor; and Joseph M. Boudreaux, former Research Associate, School of Plant, Environmental & Soil Sciences, LSU AgCenter, Baton Rouge, La.

(This article was published in the spring 2011 issue of Louisiana Agriculture magazine.)