Spring Wheat Varietal Differences to Trifluralin as Influenced by Selected Postemergence Herbicides

Trifluralin, [2,6-Dinitro-N,N-dipropyl-4- (trifluouromethyl) benzeneamine] may reduce crop stand, vegetative growth, and seed yield of wheat. High organic matter and clay content of the soil, low soil moisture, cold winters, and reduced tillage practices, and high rates increase the potential for carryover of trifluralin and subsequent stress to wheat. Further stress may result from postemergence spring wheat herbicides. The objectives of the study were to quantify trifluralin vegetative injury by variety and to determine the susceptibility of trifluralin-injured wheat to further injury from postemergence herbicides. A field study was conducted in 1991 and 1992 at Highmore and Groton, SD. Three hard red spring wheat varieties - 2375, Prospect, and Butte 86 - were seeded into areas treated with 0.56 kg ha- 1 of trifluralin immediately after incorporation. In a second study, wheat was seeded into 1.12 kg ha- 1 and 2.24 kg ha- 1 trifluralin rates that had been incorporated the previous spring and sown to soybeans. Four postemergence spring wheat herbicides - 2, 4-D (2, 4-dichlorophenoxyacetic acid), metsulfuron (methyl 2-[[[[(4-methoxy-6-methyl-1,3,5- triazin-2-yl)-amino]carbonyl]-amino]sulfonyl] benzoate), difenzoquat (l,2-dimethyl-3,5-diphenyl-lH-pyrazolium), and Tiller, a herbicide combination consisting of fenoxaprop-ethyl (2-[4-({6-chloro-2-benzoxazolyl)oxy]phenoxy]propanoate) + 2,4- D ester + MCPA ester (isooctylester/isooctyl-2-methyl-4- chlorophenxyacetate) - were applied at recommended rates and timing. In addition, a greenhouse study was conducted to determine varietal susceptibility to different rates of trifluralin. Same-year applied trifluralin and some postemergence herbicides caused early season stress to wheat and yield loss. Varieties were differentially affected by treatments. Early season stress was not correlated to yield losses. Prospect was most susceptible to trifluralin in the field study as measured by yield, followed by Butte 86 and 2375. Trifluralin injury symptoms from carryover were not observed at Groton. This study indicates trifluralin stress of wheat can be minimized by careful selection of wheat variety and postemergence herbicides. Trifluralin carryover is not likely to be a problem in northeastern South Dakota except under unusual circumstances. These circumstances may include planting wheat into trifluralin-treated soybean stubble if the growing season for the soybeans had been especially dry, late application of trifluralin the previous year, or in areas where inaccurate application or calibration has occurred