Masters Theses

Date of Award

8-1979

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant, Soil and Environmental Sciences

Major Professor

Larry S. Jeffery

Committee Members

Vernon H. Reich, Billy J. Trevana

Abstract

Common cocklebur (Xanthium pensylvanicum Wallr.) and other common weeds growing in soybean fields must be controlled in order to obtain maximum soybean yields. Many methods for achieving good weed control are available, however the cost and returns from these various systems are important, since partial weed control or even total control may or may not be economically favorable. The efficiency of weed control methods in soybeans should always be evaluated in conjunction with a detailed study of economic returns.

Field studies were conducted in Tennessee during 1975 and 1976 under conventional tillage cropping at two locations, and during 1978 under no-tillage cropping at one location. These studies were conducted to determine: (1) the efficiency of various weed control systems for control of common cocklebur and other weeds, and (2) the effects of various levels of control of common cocklebur and other weeds on soybean yields, and the cost and returns from each level of control.

In the conventional tillage experiments profluralin at 0.8 kg/ha was applied to the whole experimental area to control annual grasses and followed by sequential applications of pre and postemergence herbicides followed by between row cultivation to control cocklebur and other annual broadleaf weeds. Naptalam + dinoseb (Dyanap) at rate of 5.0 kg/ha was applied preemergence followed by postemergence applications at the second trifoliate stage of the soybeans of either bentazon at the rate of 0.8 kg/ha, or dinoseb at 0.8 kg/ha, or Dyanap at 2.5 kg/ha. These combinations were followed either by bentazon at 0.8 kg/ha or cultivation when the soybeans were 30-70cm tall, with other cultivations as needed later in the season. Also as separate treatments either linuron at 0.5 kg/ha or metribuzin at 0.6 kg/ha were applied at the preemergence stage or Dyanap at 4.2 kg/ha was applied at the cracking stage. None of these were followed by a postemergence treatment at the second trifoliate stage. These treatments were followed by either bentazon or cultivation when soybeans were 30-70cm tall plus other cultivation as needed later in the season.

In the no-tillage experiment, glyphosate at 1.7 kg/ha was applied at planting over the whole experimental area. This was followed by metribuzin at 0.6 kg/ha and alachlor at 2.2 kg/ha applied preemergence singly or mixed to each plot. Thirty days later two new postemergence herbicides coded as RH-6201 at 2.2 kg/ha and HOE-29152 at 0.6 kg/ha, were applied singly or in combination over plots previously treated with the preemergence herbicides.

Under conventional tillage all weed control methods increased yields, except Dyanap applied preemergence with no further treatment. Greatest soybean yields were produced with practices most effective for cocklebur control. The inclusion of the third treatment of bentazon or cultivation when the soybeans were 30-70cm tall followed by cultivation later in the season resulted in maximum yields. A postemergence treatment applied at second trifoliate stage of soybeans was not necessary if linuron was the preemergence treatment and bentazon was applied when soybeans were 30-70cm and followed by timely cultivation later in the season. A single post-emergence application of bentazon when the soybeans were 30-70cm tall followed by cultivation later in the season was enough to produce maximum yields, even when no prior treatment was made. (In all treatments profluralin had controlled the annual grasses.)

The predicted adjusted yields ranged from 509-1748 kg/ha with 0% control of cocklebur to 2528-3161 kg/ha with 100% control.

Total costs of production ranged from $142-167/ha with no cocklebur control to $207-228/ha with 100% control. An additional investment of $18-24/ha was required to increase common cocklebur control from 60 to 95%.

Net returns were increased with all common cocklebur control measures, except where Dyanap was applied at either the preemergence or cracking stage without a subsequent treatment. Maximum net returns were obtained in all experiments when Dyanap was applied at the cracking stage and metribuzin was applied preemergence, if these treatments were followed by bentazon applied when the soybeans were 30-70cm tall and were cultivated later in the season. A single application of bentazon when the soybeans were 30-70cm tall, followed by cultivation later in the season with no prior treatments was enough to produce maximum net returns in each experiment, except at Ames Plantation in 1976. Bentazon applied when the soybeans were 30-70cm tall followed by cultivation later in the season contributed for maximum net returns when Dyanap was applied at the cracking stage, and when metribuzin was applied in preemergence.

Failure to provide any control of common cocklebur resulted in net returns ranging from $-21 to $185/ha, according to the predicted regression equation developed to estimate the effects of these two variables. Control of 95% of common cocklebur resulted in net returns ranging from $310 to $416/ha.

According to these results the best cocklebur control systems for conventional tillage soybeans were: either Dyanap applied at the cracking stage or metribuzin applied preemergence, both followed by bentazon applied when the soybeans were 30-70cm tall followed by cultivation later in the season.

Under no-tillage cropping none of the weed control measures tested significantly increased soybean yields or net returns when compared to the untreated control. This was partially due to poor soybean stands resulting from severe rabbit damage. Predicted adjusted yields of 509 kg/ha were calculated for 0% of weed control and 1547 kg/ha for 100-6 control. Maximum weed control was provided by all herbicide combinations that included metribuzin + RH-6201. These combinations provided an average of 90% weed control.

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