Iowans are growing increasingly concerned about potential water pollution in Iowa due to heavy fertilization and extensive use of herbicides. The long-term productivity of soils has also come under scrutiny as the cumulative effects of erosion become more apparent. As a result, researchers are emphasizing the importance of developing new farming practices. One such practice already in use is ridge tillage

Jurik, Thomas W.

English

Iowans are growing increasingly concerned about potential water pollution in Iowa due to heavy fertilization and extensive use of herbicides. The long-term productivity of soils has also come under scrutiny as the cumulative effects of erosion become more apparent. As a result, researchers are emphasizing the importance of developing new farming practices. One such practice already in use is ridge tillage.</p

Jurik, Thomas

Digital Repository @ Iowa State University (ISU)

89-19 COMPETITIVE GRANTLeopold Center LEOPOLD CENTER FOR SUSTAINABLE AGRICULTURE Weed population dynamics in ridge-tilled soybeans with and without herbicides REPORT Principal investigator Thomas W. Jurik Botany Iowa State University Budget $29,356 for year one $26,721 for year two Background and goals Iowans are growing increasingly concerned about potential water pollution in Iowa due to heavy fertilization and extensive use of herbi­cides. The long-term productivity of soils has also come under scrutiny as the cumulative effects of erosion become more apparent. As a result, researchers are emphasizing the im­portance of developing new farming practices. One such practice already in use is ridge till­age. In this project, field tests on several Iowa farms showed that ridge-till cultivation of soy­beans can be an effective way to maintain high yields while reducing herbicide application and soil erosion. Ridge tillage leaves substan­tial vegetative debris on the soil surf a ce during much of the year. Thus, it is an effecti ve means of reducing soil erosion. Trials by at least one farmer suggest that ridge tillage can be used without any herbicides. In fact, weed densities with ridge tillage can be even lower than those achieved with conven­tional tillage or with minimal tillage plus her­bicides. Thus, ridge tillage is potentially very useful as a technique for maintaining both short-term and long-term soil productivity. Prior to this research, agronomists had little information on the relationship of ridge tillage to weed growth and crop yields. Researchers found in 1988 that weed seed production dur­ing a season usually replaces seed germinated during the season. They also found :hat when compared to conventional tillage with herbi­cides, ridge-tillage crop yields were reduced in continuous corn but not in corn-soybean rota­tions. Those findings suggested that crop rotation may play an important role in regulat­ing seed densities and maximizing the effec­tiveness of ridge-tillage cultivation. This two-year study built on the 1988 findings by assessing the interactions between ridge-till cultivation, pre-emergence herbicide ap­plication, weed populations, and soybean crop yield. In 1989, the objective was to contrast ridge-till cultivation with and without herbi­cide application at planting in order to deter­mine the relationship of herbicide application to environmental conditions, weed population dynamics, and crop yield. In 1990, a similar objective evaluated whether rotary hoeing was an effective substitute for herbicide banded in the row at planting. Approach and methods Four farms operated by members of Practical Farmers of Iowa were used for field studies both years, although different fields were used each year. All were located in north-central Iowa on Clarion-Webster soils. (For discus­sion purposes, we refer to the farms here by the farmers' surnames: Graaf, Grau, Hartsock, and Thompson.) Each farm used its own equipment, seed, and fertilization and herbi­cide practices to establish and grow a soybean crop on the field sites. Because a severe wind and hail storm destroyed the experiment on the Graaf farm, 1990 data are available only for the other three farms. While the herbicide and fertilization practices varied between sites, they were consistent among treatments on a site. Farmers banded herbicide in the row at planting in those treat­ments receiving herbicide. They replicated Leopold Center Progress Reports 41 each treatment on each site at least six times, using a complete randomized block design with each of two (in 1989) or three (in 1990) treatments constituting a block. Each plot had six or eight rows, either 400 or 800 meters (440 or 880 yards) in length. In 1989, farmers either banded herbicide in the row at plantir g or used no herbicide. All plots received two rotary hoeings and two ridge-building cultivations. All were cultivated twice. On the Thompson farm, heavy rains the day after planting caused extensive washing. Be­cause the herbicide movement and loss was probably substantial, the trials at this location were converted to test the effect of rotary hoeing on weed populations. Those plots that originally received herbicide were rotary-hoed once; the others were rotary hoed four times. Both were cultivated twice. In 1990, farm cooperators essentially used three experimental treatments: (1) no herbi­cide, two rotary hoeings, two cultivations; (2) herbicide banded in a row, one rotary hoeing, two cultivations; and (3) herbicide banded in a row, no rotary hoeing, two cultivations. (An extremely wet spring caused late planting and delays in other field operations, resulting in one rotary hoeing and three cultivations on the Grau and Hartsock farms.) Weed population studies: Weed populations were usually sampled at irregular intervals, from late April to early August, a day or two before a field operation. Samples were taken nine times in 1989 and 12 times in 1990. Because cultivation made it impossible to re­locate exactly the same position in the field each time, investigators targeted a sample area for each plot that was typically from four to twelve square meters (43 to 129 square feet) in size. In late summer, however, low weed densities allowed an increase in size of some sample areas to as large as 600 m2 in each plot, or nearly 10% of the total plot area. Investigators recorded the number of weed seedlings, non-fruiting adults, and fruiting adults of all weed species present. Plants were further subdivided according to position in the ridge-furrow topography (see Fig. 1). The row area was defined as a 20-cm-wide strip (about 8 inches) with the soybean plants in the center. The row shoulder was a 15-cm-wide strip; the furrow width varied from 26 to 50 cm on the various farms. Weed counts on the two row shoulders in each sample were combined. Environmental measurements: Coopera­tors determined the amount of plant residue aboveground in late April 1989 by collecting Fig. 1. Row, row shoulder, and furrow positions studied in the ridge-till system. Volume 2 (1993) 42 the biomass from quadrats (sample areas) in each plot. One quadrat was centered on the row; the other was centered on the furrow. (Shoulders, which had very little plant cover, were not sampled.) Soil was sampled for nutrient content. Soil temperature and mois­ture content were monitored continuously in the field. Farm cooperators also recorded rainfall data. Monitoring of soil moisture content, soil temperature, and rainfall contin­ued in 1990. Microenvironmental study: In ridge tillage, three "microsites" experience different envi­ronmental conditions and degrees of distur­bance by cultivation (Fig. 1). The planter removes the top 5 cm from the ridge and transfers the soil to the furrow. Later ridging operations move the soil from the furrow back to the row, creating the ridge. The row shoul­der is little affected by either the planting or ridging operations. Thus, weeds are not easily removed from this area by mechanical means. On the Thompson farm, microenv uronmental conditions and their relation to residue cover were studied intensively. In April 1989, the investigator used three treatments: normal amount of residue, residue removed, and twice the normal amount of residues (created by placing all residues removed from a 10-m by 10-m plot onto an adjacent one of identical size). This test was replicated once: in each of three plots with herbicide and three without. Heavy rains in late May caused ponding and subsequent movement of the residue in two sets of the treatments, but measurements were taken in the remaining set until late July. Temperature and light were recorded at 5- or 10-minute intervals; soil temperature was measured for two row, shoulder, and furrow positions in each plot. Instrumentation also measured light level above the plant canopy and at the soil surface in two row, shoulder, and furrow positions (under the litter). In addition, weed photosynthesis was mea­sured in the field, although small weed size delayed these measurements until late July. Leopold Center Progress Reports Findings In 1989, mid-July soybean plant densities were about 80% of the seeding density on all but the Graaf farm, where plant density was 53% of seeding density. In 1990, soybean plant den­sities in early August were 45 to 74% of the seeding densities. Rainfall in 1989 was lower than normal on all farms, though the Thomp­son farm had nearly twice that of the Hartsock farm. Although low, the rainfall was distrib­uted fairly evenly over the growing season on all farms. Thus, soybeans exhibited few symp­toms of moisture stress. In 1990, total rainfall was above normal for the season but was unevenly distributed, with higher amounts earlier in the season. Weed population dynamics: Thirteen major weeds were found over the growing season. Redroot pigweed, lambsquarters, foxtail spe­cies, and Pennsylvania smartweed were most common. In general, the same species were found on a given farm in both years, although some variation occurred because different fields were used each year. Patterns were similar across all farms. In 1989, almost no seedlings were found in any plot in late April; in 1990, very few seedlings had appeared even by late May. In 1989, the total number of weed stems was highest in May and declined over the summer. There was relatively little differ­ence in weed numbers between plots with and without herbicide, particularly later in the sea­son. Weed seedlings appeared in greatest numbers in May and early June; practically no seedlings appeared after the last cultivation in late June or early July. Adults present late in the season arose almost entirely from seed­lings appearing before the last cultivation. The data clearly showed that rotary hoeing and cultivation were highly effective in reducing weed numbers. In the 1989 trial of one rotary hoeing versus four on the Thompson farm, total weed num­bers were far higher in the former. There were more than ten times as many weeds in the one rotary hoeing plot, primarily because large numbers of seedlings established themselves in June and were not removed by later cultiva­43 For more information contact T. W. Jurik, Botany, Iowa State University, Ames, Iowa, 50011, (515)294-5617. tion. Proportional differences in numbers of adult plants between the treatments generally increased over June. In the plots that had been rotary hoed four times, total weed numbers in late July were similar to weed numbers in the with/without herbicide treatments on the other farms. Positions in the ridge-furrow topography dif­fered in number of seedlings and adults present. Results for the herbicide trials resembled those for the rotary hoeing trials. The shoulder generally had the greatest number of seed­lings, while the furrow had the fewest during May and June. The same was true for adult plants. Yet by late July, numbers of adults were highest in the row and lowest in the furrow. Most late-summer weeds grew in the row area, where they were less likely to be destroyed by cultivation. Maximum weed populations in June were about the same in the plots with and without herbi­cide on the Graaf, Grau, and Hartsock farms. On the Thompson farm, weed populations in the plots hoed four times were similar to those in the herbicide trials on the other farms. Weed numbers in the treatment hoed once were much higher. Cultivation reduced weed numbers in late July to less than 0.5% of those in early June in all but the treatment hoed once. In that one, cultivation still greatly reduced weed num­bers. Soybean yield in 1989 did not differ signifi­cantly between plots with or without herbicide on the Graaf and Grau farms. The Hartsock farm plots showed small yield increases with­out herbicide. On the Thompson farm, the plots hoed four times had significantly higher yields than those hoed once. Redroot pigweeds on the Thompson farm pro­duced between 150,000 and 270,000 seeds per plant. (A rough estimate suggests these weeds produced between 40 million and 400 million total seeds per acre!) Weed population dynamics in 1990 resembled those of 1989. Maximum weed populations in June varied widely among farms. The Grau farm had unusually low numbers in all treat­ments; cultivation resulted in reasonably low numbers on all farms. Soybean yield did not differ significantly among treatments on any of the farms. Environmental conditions: Soil moisture in 1989 greatly declined over the season on all but the Thompson farm. By June, moisture level differed little by row position. Soil temperature was largely a function of sam­pling time. After planting, temperature dif­fered little by row position. The resulting data also include local weather conditions, month, soybean canopy, cultivation, and residue. Redroot pigweed's photosynthetic rate was among the highest reported for any species. This may explain pigweed's fast growth and ability to overtop soybean. This weed played a major role in reducing yields on the Thomp­son farm plots. Implications The results of this study indicate that careful management can replace herbicides for weed control in ridge-tillage soybean production. A large part of the efficiency of ridge tillage in controlling weeds appears to come from use of the rotary hoe. In fact, rotary hoeing followed by cultivation achieved weed control compa­rable to that of herbicides in this same system. Thus, ridge tillage can help protect long-term productivity and avoid further environmental degradation by reducing the amount of herbi­cide used to control weeds. The optimal amount, timing, direction, and other aspects of rotary hoeing still need to be determined. Future research could help to optimize this technique for controlling weeds, reducing the use of fossil fuels and manage­ment inputs, and maximizing crop yields. The participation of Practical Farmers of Iowa was critical to the success of this project. It also provided opportunities to disseminate the re­sults of this study via field days and newslet­ters. Volume 2 (1993) 44 

Weed population dynamics in ridge-tilled soybeans with and without herbicides

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Weed population dynamics in ridge-tilled soybeans with and without herbicides

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