Influence of Cultural Practices on Weed Management in Soybean

Weed management in soybean [ Glycine max (L.) Merr.J is influenced by cultural practices including planting date, fertilizer and herbicide application, and cultivation. Field studies were conducted in 1990, 1991, and 1992 to examine the interactions of planting date, deep‐banded fertilizer, banded and broadcast herbicide applications, rotary hoeing, and cultivation on weed management in soybean. Soybean was planted in early May, with or without deep‐banded fertilizer, and planted in late May without fertilizer. Nineteen weed management treatments were included in the study, ranging from mechanical control only to broadcast herbicides followed by rotary hoeing and cultivation. Weed density, soybean population, and soybean yield were measured. Gross margin over weed control cost was calculated for each weed management system. Applying deep‐banded fertilizer at planting did not influence weed densities, soybean population, or soybean yield. Rotary hoeing 4 to 6 d after planting reduced soybean populations 9 to 15%, while rotary hoeing 12 to 21 d after planting did not reduce soybean populations. Rotary hoeing did not reduce weed densities or improve soybean yield when herbicides were broadcast or band‐applied. In the absence of herbicides, timely rotary hoeing followed by two cultivations reduced weed density and increased soybean yield to within 75% of the hand‐ weeded control. Cultivating broadcast preemergence and postemergence herbicide applications increased soybean yield and gross margin in eight of 10 instances. Soybean yield and gross margin over weed control costs was greatest where postemergence herbicides were broadcast and soybean cultivated once, followed by banded postemergence herbicide application and soybean cultivated twice. Soybean yield was greater when planted in early May than late May in 89% of the weed management treatments in 1991 and 78% of the weed management treatments in 1992. Therefore, planting early, broadcasting or banding postemergence herbicides, and cultivating resulted in more consistent weed control and higher gross margins than other weed management strategies. Research Question The Clinton administration has committed to efforts in research and education to achieve adoption of integrated pest management on 75% of the crop acreage in the USA by the year 2000. Strategies of integrated weed management include changing cultural practices such as crop planting date or fertilizer application, reducing herbicide use, and increasing mechanical weed control. The objective of this research was to determine whether weeds could be managed with reduced herbicide inputs in early‐ and late‐planted soybean in the northern USA. We were interested in finding whether deep‐banding fertilizer at planting would increase weed density or soybean yield, and whether delaying soybean planting until late May would reduce soybean yield and gross margin in various weed management systems. Literature Summary Broadcast fertilizer applications increased weed populations in sugar beet, but deep‐banded fertilizer applications decreased weed populations and increased crop yield compared with broadcast fertilizer applications. Delaying crop planting may improve weed control because many weeds, will germinate and be controlled by tillage prior to planting, but delayed planting may be detrimental to crop yield. Weeds were more competitive in early‐planted soybean in Minnesota, and more competitive in late‐planted soybean in Arkansas research. The effect of delayed planting on percentage yield loss due to weed interference in northern climates is not fully understood. Banded herbicides followed by rotary hoeing and two cultivations resulted in good weed control and crop yield and high gross margins in corn and soybean. Mechanical weed control alone resulted in soybean yield equal to that where herbicides were applied when weed pressure was low (<10 weeds/sq ft). Study Description Field experiments were conducted in 1990, 1991, and 1992 on a loam soil with a pH of 7.3 to 7.8 at East Lansing, MI. ‘Elgin 87’ soybean was planted in 30‐in. rows in early and late May. The experiment was a randomized complete block split‐plot with a factorial arrangement of planting date, fertilizer, and weed control treatment combinations. The whole plots were (i) early May‐planted soybean with no starter fertilizer (6‐24‐24), (ii) early May‐planted soybean with 198 lb/acre of starter fertilizer deep‐banded 2 in. to the side and 2 in. below the soybean seed, and (iii) late May‐planted soybean with no starter fertilizer. The 19 weed management treatments were the sub‐plots and included: a weed‐free control, a nontreated control (no herbicide or mechanical control), three mechanical control treatments, and 14 treatments that included herbicide. Herbicide was applied preemergence or postemergence as a broadcast or 10‐in. banded treatment. Broadcast treatments were either (i) not rotary hoed or cultivated, (ii) not rotary hoed and cultivated once, (iii) rotary hoed and cultivated once, or (iv) rotary hoed late and cultivated once. Banded herbicide treatments were either (v) not rotary hoed and cultivated twice, (vi) rotary hoed once and cultivated twice, or (vii) rotary hoed late and cultivated twice. Metolachlor + linuron + metribuzin was the preemergence herbicide program, while bentazon + acifluorfen was the postemergence herbicide program. The standard rotary hoe timing was 4 to 6 d after planting, when weeds had germinated but not yet emerged. Treatments were cultivated when weeds were approximately at the four leaf stage, 4 to 5 wk after planting. Soybean was harvested for yield and gross margin over weed control costs was calculated by multiplying the soybean seed yield by the assumed market price of $5.50/acre and subtracting herbicide, application, and mechanical weed control costs. Applied Questions What was the effect of deep‐banded fertilizer? Deep‐banded fertilizer did not influence soybean population or soybean yield in 1990 or 1991. Soil pH at this site was above 7.0, and fertility was high so we did not expect a soybean response. Deep‐banded fertilizer did not increase in‐ or between‐row densities of small‐seeded broadleaf weeds. What was the effect of planting soybean in late May? Delaying soybean planting did not reduce weed populations as expected. Soybean yield and gross margin were greater when soybean was planted in early‐ compared with late‐May for most of the weed management treatments. Were weeds controlled without applying herbicides? Soybean yield was reduced 17 to 23% in the early‐May and 16 to 21% in the late‐May planting when weeds were partially controlled by rotary hoeing and cultivation only. Weed populations in this research were >20 weeds/sq ft for each planting date in each year. Mechanical weed control has been successful in soybean when weed populations were low (<10 weeds/sq ft) and grasses were not dominant. Were weeds controlled by banding herbicides and cultivating? Banding postemergence herbicides and cultivating twice was a consistent weed management program in 2 of 3 yr. Preemergence banded herbicide treatments were not as consistent. What effect did rotary hoeing have on weeds and soybean? Rotary hoeing 4 to 6 d after soybean planting reduced soybean stand 9 to 15%. Rotary hoeing was not necessary when herbicides were broadcast preemergence or postemergence and soybean cultivated once. Additonally, rotary hoeing was not needed when herbicides were banded and soybean cultivated twice. Was cultivation of broadcast herbicide applications necessary? Cultivation reduced weed densities and increased soybean yield and gross margin 80% of the time. Integrated weed management systems that include herbicides and cultivation have been the most reliable means of controlling weeds in many crops, including soybean. What weed management system resulted in the highest gross margin? Gross margins over weed control costs were consistently greater where postemergence herbicides were broadcast and soybean cultivated once, followed by banded postemergence applications and two cultivations. Recommendation Based on this study, we recommend timely planting of soybean in northern production regions. When weed densities exceed 20 weeds/sq ft, herbicide(s) should be applied followed by cultivation. Postemergence herbicides provided more consistent weed control than preemergence herbicides. Yield and gross margin in soybean where herbicides are banded can equal that of broadcast herbicide when cultivation is timely.