Will Certain Adjuvants Improve the Preemergence Activity of Pendimethalin or S-metolachlor?

Aims: To determine weed efficacy when using adjuvants such as Grounded or Spectrum in combination with pendimethalin or S-metolachlor.

Study Design: Randomized complete-block with 3 replications.

Place and Duration of Study: Field studies were conducted during the 2021 growing season near Yoakum (29.2765o N, 97.1238o W) in south-central Texas and near Corpus Christi (27.7817o N, 97.5737o W) along the upper Texas Gulf Coast.

Methodology: Herbicides were applied preemergence either alone or in combination with Grounded at 2.3 L ha-1 or Spectrum at 0.6 L ha-1 after corn (Zea mays L.) or peanut (Arachis hypogaea L.) were planted at Yoakum and under a non-crop situation at Corpus Christi.

Results: In corn when evaluated 7 weeks after treatment (WAT), the addition of Grounded to pendimethalin at 1.06 kg ha-1 reduced Palmer amaranth control when compared with pendimethalin alone at the same rate or with the addition of Spectrum. Smellmelon control with pendimethalin at 0.53 kg ha-1 improved with the addition of Grounded over pendimethalin alone. When evaluated 15 WAT, only Texas millet was present in consistent enough populations to evaluate and no differences in control were noted with/without an adjuvant with either pendimethalin or S-metolachlor. In peanut when evaluated 26 and 54 days after treatment (DAT), Texas millet or smellmelon control was not influenced with the use of either Grounded or Spectrum. In the non-cropland study, when evaluated 14 DAT, both Texas millet and Palmer amaranth control was > 97% with all combinations of pendimethalin or S-metolachlor with/without an adjuvant. At the 42 DAT evaluation, again no differences in weed control were noted between the herbicides without an adjuvant and with the addition of Grounded or Spectrum.

Conclusion: In none of the trials did the addition of either Grounded or Spectrum to either pendimethalin or S-metolachlor consistently improve weed efficacy. Also the length of herbicide persistence was not increased with these adjuvants.