Pre-Plant Weed Burndown in Soybeans

William Hogan  |  4/5/2011 7:53:10 PM

Pre-plant Weed Burndown in soybeans is a production practice that has been more common in other parts of the state than in Southwest Louisiana. There are several reasons for increased interest in pre-plant burndown. It can have advantages over traditional tillage.

Tillage is the traditional method used to destroy existing vegetation before planting. This seed bed preparation method has been used with varying degrees of success as long as crops have been grown. While burndown herbicides and their application are not free, neither is diesel nor operator expense involved in tillage. The usual tillage procedure involves disking, followed by a rain, wait for the soil to dry and repeat the cycle over and over until planting time. Each time over the field is an additional expense. In addition, after several repetitions of the till-rain-till cycle, our prairie soils can develop an extremely compacted structure not unlike cured concrete (I have exaggerated, but not much). The roots of the previous crop or the even the weeds that were present have helped to loosen the soil. The more you travel over it, the tighter it packs. This can leave a situation that is not ideal for soybean root penetration or rhizobium nodule formation.

Sometimes, a producer is unable to begin tillage until the weeds present are large and have developed thick soil surface cover. Even if through tillage destroys these plants, they are incorporated into the plant bed and will take time to decompose. Unless decomposed by planting time, they can cause physical problems for a grain drill and result in uneven soybean plant populations. If the existing weeds are destroyed by a burndown herbicide application and allowed to desiccate before planting, this much less likely to occur. Dead, dried weeds are much easier to no-till plant into or to destroy with a single tillage before planting. If it works as it should, you have saved trips over the field, planted into a weed-clean field and provided for a better growing environment for the soybean plants.

There are several herbicides labeled for pre-plant weed burndown in soybean. Included in this group (not a complete list): glyphosate; gramoxone; Aim; Ignite; Harmony; Valor; Canopy; Sharpen; Reflex; Clarity; 2,4-D; Envive and Enlite. Be sure to read the label directions for the weeds controlled, weed height limitations, and use rate of each. Some may be used alone. Some should be used in combination with others. This will depend on the weed species present and the size of the weeds at application time. Many have a required time interval between application and planting. The planting interval may vary for the same herbicide depending on weather conditions, soil pH and/or the rate applied. Some have residual activity against certain weeds. Others don’t. As I said, be sure to read the label and follow all directions and restrictions.

Glyphosate (in its many formulations and brands) is widely used as a weed burndown herbicide. Often using it in combination with another burndown herbicide will be even more effective, depending on the weed species present. Using combinations of herbicides with different modes of action can help prevent herbicide resistance in a given weed species. Herbicide resistance has not been as common a problem in Southwest Louisiana as in other areas. Let’s try to keep it that way.

This spring of 2011 we have established a pre-plant soybean weed burndown demonstration on the Berken Farm in Lacassine. Our herbicide treatments are glyphosate alone; glyphosate plus Sharpen; glyphosate plus Valor; and glyphosate plus 2,4-D. Each treatment area is 10 to 15 acres in size. The weed species include maturing cool season species and immature warm season weeds. I will share the results upon completion of the demonstration.

Tree Caterpillars
One day last week, I found a large group of these in a cluster on the trunk of the Nuttal oak tree next to our office. This tree has served as my sentinel tree for this pest for several years. I know that when the forest tent caterpillars appear on it, they have begun to show up on other trees in the area. These are hairy caterpillars that have a line of diamond-shaped spots running down the length of their back. They usually travel in a line, one after the other, on the tree limb. They feed on the leaves of the tree. They can strip the leaves from a small tree in a very short period of time. Since their feeding occurs in the early spring, even a defoliated tree will re-sprout new leaves and be back to normal appearance in a few weeks. Their feeding will reduce the overall vigor of the tree. These caterpillars will soon become pupa and then adults. Thankfully these life-stages don’t feed on trees. Forest tent caterpillars don’t have the ability to sting people.

The caterpillars that can sting also feed on trees. Buck moth caterpillars, Io moth caterpillars and Pus moth caterpillars can and will cause painful stings. They are covered with spines that break off in your skin and release a protein causing pain. Most people are stung when they stand under a tree. The caterpillar will fall from the tree and land on the person’s neck or arm. Occasionally, a small child will pick up stinging caterpillars out of curiosity. The Buck moth caterpillar is the most common stinging caterpillar. It is 2-3 inches long and covered with coarse, dark, stiff spines. I recently saw a Buck moth adult that was attracted to light at a store. The eggs will soon be hatching and caterpillars will be present. The Io and Pus moth caterpillars are much less common. The Io caterpillar is short, stubby and covered with fine hairs. It has a saddle-like spot on its back. The Pus moth caterpillar looks like a hair ball that a cat spit up.

There are several insecticides that will control any or all of these caterpillars. These include Malathion, Carbaryl, Orthene, any of the B.T. materials and several synthetic pyrethroids that are sold under a company name. Always read and follow the label directions when using any pesticide.

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