The Efficacy of some Herbicides and Cultivation on Weed Control and Rapeseed (Brassica napus L.) Yield

Rapeseed (Brassica napus L.) represents considerable diversity in vigorous potential depending on field management practices, such as herbicides application which is an effective option in rapeseed production. Herbicides types determine time and application method; thereby utilizing different types of herbicides influences competitive ability of rapeseed against weeds and subsequently alters the outcome of the competition depending on agro-ecological conditions and existent weed populations. Moreover, response to different type of herbicides can vary among weed populations, and one or more population might be more constrained, which would change the weed population dynamics. This experiment was conducted to evaluate the efficacy of some preplant, preemergence and postemergence registered herbicides (in Iran) besides cultivation on weed control and rapeseed yield.

A randomized complete block design with 20 treatments each in three replications was established in Mahidasht region of Kermanshah Province. The treatments included preplant soil incorporated application of trifluralin+ postemergence application of haloxyfop (treatment 1), preplant soil incorporated application of trifluralin+ postemergence application of sethoxydim (treatment 2), preemergence application of pendimethalin + postemergence application of haloxyfop (treatment 3), preemergence application of pendimethalin + postemergence application of sethoxydim (treatment 4), postemergence application of clopyralid + postemergence application of haloxyfop (treatment 5), postemergence application of clopyralid + postemergence application of sethoxydim (treatment 6), postemergence application of clopyralid + postemergence application of cycloxydim (treatment 7), postemergence application of clopyralid + postemergence application of clethodim (treatment 8), postemergence application of clopyralid + postemergence application of cycloxydim quizalofop (treatment 9), postemergence application of clopyralid + postemergence application of fluazifop (treatment 10), postemergence application of haloxyfop (treatment 11),postemergence application of sethoxydim (treatment 12), postemergence application of cycloxydim (treatment 13), postemergence application of clethodim (treatment 14), postemergence application of quizalofop (treatment 15), postemergence application of fluazifop (treatment 16), one (treatment 17) and two cultivation (treatment 18), and weed infested (treatment 19) and weed free (treatment 20) conditions.

The results revealed that the clopyralid application with grass herbicides effectively reduced both density and dry weight of broadleaf weeds. Clopyralid application along with haloxyfop-methyl decreased wheat (as a weed in canola, and the prior crop in the field) density and dry weight as much as weed-free conditions. Applying clopyralid with other grass herbicides also resulted in significant reduction compared to weed infested control. Application of trifluralin and pendimethalin with each of haloxyfop and sethoxydim was not significantly different in terms of density and dry weight of broadleaf weeds and total weeds (total weed-plants except wheat), and also density of total weed-plants (including wheat). Thus, they caused substantially reduction in density and dry weight of broadleaf weeds and total weed-plants. However, they were not as effective as weed free treatment. As compared with sethoxydim, application of Haloxyfop accompanied by either trifluralin or pendimenthalin was more effective for wheat control (as a weed); as density and dry weight of wheat and dry weight of total weed-plants in treatments of trifluralin along with haloxyfop were significantly lower than with sethoxydim. Using acetyl coenzyme A carboxylase inhibitors alone which did not control the broad-leaf weeds properly. The haloxyfop caused a higher decrease in density and dry weight of wheat (as a weed), while the least impact was found for fluazifop. The effect of twice cultivation on weed density and dry weight was not significantly different from once cultivation, therefore, weed density and dry weight were higher than weed free condition. However, they were less than weed-infested condition. In general, Although none of the treatments were able to decrease total weed-plants (including wheat as a weed) density and dry weight up to the weed-free condition, but applying clopyralid with grass herbicides considerably reduced the total weed density (total weed-plants minus weeds) as well as the level of weed-free conditions. For canola traits, hand weeding treatment showed superiority to the other treatments. The magnitude of loss occurred in plant height, biological yield, grain yield, harvest index, Silique number per plant, grain number per plant, 1000-grain weight and grain oil percentage was 26, 48, 81, 63, 59, 6, 51, and 5 %, respectively. The maximum reduction in rapeseed measured traits was observed to be 7 % for clopyralid applied along with haloxyfop, 11 % for quizalofop, 16 % for fluazifop, 17 % for clethodim, and 20 % for the other grass herbicides. Using trifluralin and pendimethalin with each of haloxyfop and sethoxydim was not significantly different from the individual application of acetyl coenzyme A carboxylase inhibitors; and consequently it does not appear to be an appropriate option.

Weed competition not only reduces grain yield, but also grain oil (5% loss). Since a yield reduction between 5% and 10% is acceptable with herbicide application, it is possible to utilize clopyralid along with haloxyfop-methyl in rapeseed fields particularly where wheat has been previously cultivated.