Regina P. Bracy and Richard L. Parish

Difficulty in obtaining and maintaining an adequate stand in direct-seeded cabbage led to the investigation of possible culprits such as herbicides, insects and diseases. We hypothesized that use of Treflan (trifluralin) during periods of high soil temperature and low soil moisture would reduce germination and seedling vigor.

A series of experiments was conducted evaluating the effect of trifluralin (Treflan 4E) at rates of 1/2, 3/4, 1, 1 1/2, 2 pints per acre on cabbage. The recommended rate for fine sandy loam soils in this area is 1-1 1/2 pints per acre.

The herbicide was applied to prepared, weed-free beds and incorporated into the top 2 inches of soil before seeding cabbage. The cabbage was seeded into twin drills on a 48-inch bed with a Stanhay belt seeder. Seed spacing was nominally 8 inches apart.

To determine if the herbicide was causing injury to the seedlings, activated charcoal was used to neutralize the harmful effects of the trifluralin herbicide applied at the higher rates of 1 1/2 and 2 pints per acre. Activated charcoal as a slurry was sprayed at the rate of 300 pounds per acre directly into the seed furrow (after seed drop but before seed covering).

Half of the planted area was irrigated with overhead sprinklers, and the other half was irrigated with furrow irrigation immediately after planting. Weekly counts of live cabbage plants were made after seedlings had emerged (ground crack). Weed counts of grass, broadleaf and sedge weeds were taken about eight weeks after planting from a representative section of each plot.

Herbicide rate did not have a significant effect on plant stands when furrow irrigation was used. However, when plots were sprinkler-irrigated and treated with trifluralin at the higher rates of 1, 1 1/2 and 2 pints per acre, significantly fewer cabbage plants were recorded one, two, three and four weeks after emergence than on plots treated with zero or 1/2 or 3/4 pints per acre of trifluralin (Figure 1). Treating the plots with 1/2 or 3/4 pint per acre of trifluralin did not reduce the plant numbers compared with using no herbicide.

More plants were noted on the plots where activated charcoal was applied to the seed furrow before planting even though high rates of trifluralin also were applied (Figure 2). Since charcoal neutralizes herbicide effects, these results verify that plant losses were caused by increasing trifluralin rates.

Control of grass, broadleaf and sedge weeds was similar with the use of 1/2 and 3/4 pints per acre of trifluralin as it was in plots receiving the higher rates of 1, 1 1/2 and 2 pints per acre. Using no herbicide on the plots resulted in higher numbers of grass and broadleaf weeds.

Since plant losses were less and weed control was as good with the lower rates of trifluralin, LSU AgCenter recommendations for trifluralin rates for cabbage have been reduced from the labeled recommendation of 1-1 1/2 pints per acre to 3/4 pint per acre. Especially on light-textured soils, producers should not use more than 3/4 pint per acre of trifluralin on direct-seeded cabbage or reduced stands can be expected.