Controlling a legume weed in a legume crop is an added challenge.
By B.Colvin, J. Ferrell and R. Leon
Hairy indigo (Indigofera hirsuta) is an annual legume that was introduced to Florida as a forage crop. It has since escaped cultivation where it can be a troublesome weed in some crop settings. Hairy indigo is particularly difficult to manage in peanut production since we are attempting to control a legume weed in a legume crop.
Hairy indigo germinates in late spring and continues throughout the summer. In general, it is the later-emerging ones, those in late May through June, that are often the most problematic since many of the postemergence herbicides have already been applied.
Competition And Interference
Hairy indigo commonly grows between two and five feet in height, and the stem becomes increasingly woody with age. As the name would suggest, the leaves are covered with a very dense mat of fine hairs that increase in thickness with age.
Hairy indigo in peanut can reduce yield in two ways. 1. The plant gains a significant height advantage over peanut and forms a dense canopy. This reduces photosynthesis, but also intercepts fungicide and leads to increased disease incidence. 2. The woody stems of hairy indigo complicate the peanut digging process and significant peanut yield loss can result.
Few soil-applied herbicides have been found to adequately control hairy indigo. This, plus the fact that the weed has a long germination window, generally means that preemergence herbicides are of marginal value on hairy indigo. Therefore, postemergence herbicides were tested to determine which program would adequately control this plant.
Experiments conducted at the University of Florida tested five herbicide combinations on hairy indigo that was one to two inches, two to four inches and four to six inches in height. All herbicides were applied with a crop oil at one percent volume/ volume.
Plant Height Equals Big Difference
Hairy indigo control, when sprayed at the one-to two-inch stage, was acceptable for all herbicides tested. Cadre, alone, provided a modest 78 percent control, but the Cobra and Storm treatments all provided 90 percent control or greater.
Surprisingly, allowing the plants to reach two to four inches in height dramatically impacted the ability to control this weed. This small difference in plant height reduced control by 12 to 32 percent for all treatments.
As the plants reached four to six inches in height, the Cadre treatments were decreased yet again to 30 to 42 percent. However, control from the Cobra and Storm treatments stayed relatively stable as height increased from two to four inches to four to six inches.
Leaf Hairs Intercept Fungicides, Too
This rapid reduction in herbicide activity was somewhat expected since the density of leaf hairs increases rapidly as the plants pass out of the seedling stage. These thick hairs intercept the herbicide and prohibit the droplet from absorbing into the leaf.
The data indicates that hairy indigo is a weed that can be managed with our current peanut production practices. However, if this weed has been a problem in the past, it is essential that we scout these fields often and prepare to spray when the weeds are small.
It must be noted that applications of Cobra should be used with caution if the peanut crop is greater than eight weeks old. Numerous trials have shown that yield reduction will commonly occur if Cobra is sprayed at this time. If hairy indigo is present at this late-season time, it would be advisable to use Storm plus 2,4-DB to manage the weed.
Article written by Jason Ferrell, professor, UF/IFAS Agronomy Department; Blaire Colvin, graduate assistant, Agronomy Department; and Ramon Leon, assistant professor, UF/IFAS West Florida Research and Education Center, Jay, Fla.