Illinois Fertilizer
Conference Proceedings
January 23-24, 1990
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W.S. Curran1
Herbicides are an integral part of controlling weeds in today's agricultural setting. In Illinois, well over 90 percent of our corn and soybean acreage receives one or more herbicide applications (Pike, 1988).
With increased attention toward human health and the environment, pest management strategies that rely on chemical inputs have come under fire. This paper will review some of the benefits and risks that herbicides offer to Illinois row crop production and discuss how these compare to less chemically intensive systems.
Prior to the introduction of synthetic herbicides, producers relied principally on cultural and mechanical weed control methods. Rotation was an important part of Illinois crop production. Typical rotations in Illinois often included a small grain and/or forage legume in rotation with corn. With the widespread adoption of the soybean, the reduction in livestock numbers and the introduction of synthetic pesticides and fertilizers, rotations that included small grains and hay decreased.
Crop rotation is a valuable tool for breaking weed life cycles and allows for the application of alternative control measures. The more dissimilar the crops and their management practices in a rotation system, the less opportunity an individual wed or other pest has to become dominant. Without rotation, individual species or groups of species have a better chance to proliferate and spread.
In today's complex world market and government farm policy, it is unclear how crop rotation can or will play a role in pest management.
Other cultural practices widely used prior to World War II included later planting dates to allow early weed germination followed by subsequent tillage, leaving ground fallow with repeated tillage, especially for perennial weed control. Also, planting winter cover crops helped conserve soil and suppress weed growth.
By far, the most common weed control method prior to the adoption of herbicides was tillage. Tillage in Illinois often included a combination of plowing, disking, harrowing, rotary hoeing, field cultivating and row cultivating. Frequent and repeated cultivations were the most efficient method of weed control in corn as well as other row crops. An early circular developed by International Harvester states, "Black surface attained by frequent cultivations that keep all sprouts cut off at or below the soil surface is most successful and economical" (2). This was particularly advocated for perennial weed control.
With the introduction of 2,4-D in 1944, Randox in 1955, Treflan in 1964 (Slife, 1973), and a whole host of compounds today, we have moved from a tillage-intensive farming system to a chemical-intensive one. Herbicides have changed the way we farm and provided a number of important benefits.
They allow us to control weeds where row cultivation is impossible and reduce the number of tillage operations as well as the critical timing needed for operations. This has allowed the widerspread adoption of conservation tillage practices. Additionally, herbicides have reduced the amount of human labor necessary for hand weeding, controlled weeds not economically or feasibly controlled by other methods and allowed greater flexibility in the choice of management systems.
However, herbicides are not without problems and the tendency of many practicing agriculturalists to expect that any and all weed problems can be solved effectively with herbicides has contributed to this.
These disadvantages are often termed "externalities" and are difficult to identify and more difficult to quantify or put a price tag on. Some of these problems include injury to nontarget species, crop injury, herbicide carryover, container disposal problems and more recently, surface and groundwater contamination and the associated potential health risks from human exposure. These environmental and human health risks are responsible for the reevaluation of pesticides that is currently taking place in the U.S.
In contrast, without synthetic herbicides, both benefits and risks also exist. Without herbicides, the most visible and publicized benefit is the reduction in human exposure and elimination of potential adverse environmental effects.
The disadvantages are less clear, although discussions typically center on the potential reduction in yield and crop quality, and the overall effect this might have on price and availability.
In general, herbicides have been a direct substitute for tillage and human labor. When herbicide inputs are reduced or eliminated, other inputs must be substituted back into the system if adequate weed control is to be maintained. Cultural practices and mechanical control measures are currently the most effective alternative to herbicides. Rotation, cultivation, rotary hoeing, altering crop density and planting date and mulching are some of the practices commonly employed.
Cultivation, if performed in a timely fashion, can contribute much to a weed control program. In fact, 70 percent of Illinois corn acres and 46 percent of Illinois soybean acres still receive one cultivation (Pike, 1988). Growers who use little or no herbicide generally rely heavily on mechanical weed control for corn and soybeans. These producers often rotary hoe once or twice shortly after planting and follow this up with at least two cultivations.
Research at the University of Illinois has compared two cultivations with and without soil-applied herbicides in corn. Preliminary results show that the best yields in 1989 were achieved with a combination of herbicides and cultivation, followed by herbicides with no cultivation. Two cultivations with no herbicide achieved the lowest yield (Table 1).
Annual grass and broadleaf weed pressure were quite severe in this study. Much of the success of cultivation will depend on what weeds are present, how severe infestations are and timeliness of control.
The economics of comparing herbicides versus mechanical control can be very complicated. U of I agricultural economists estimate the cost of rotary hoeing and row cultivation are $1.75/acre and $6.25/acre, respectively. A simple comparison of one or two rotary hoeings, followed by two or three cultivations versus the cost of a typical herbicide application and no cultivations to corn and soybeans, shows the total cost can be similar in the two systems (Table 2).
However, by reducing our tillage or chemical input (one less cultivation or by banding our herbicide), the total cost can vary considerably. We are also making the assumption that weed control is comparable in the two systems, which may be incorrect depending on prevalent weed species and weed density.
Two other factors should be considered when evaluating tillage versus herbicides. First, since the introduction and widespread adoption of herbicides, several weed species have been introduced or have become more widespread in Illinois. Giant foxtail was introduced into the U.S. from China in the 1930s and spread rapidly throughout the Midwest by the 1950s (Knake et al., 1971). Prior to this time, farmers did not have giant foxtail to contend with.
Perennial weeds, such as hemp dogbane and johnsongrass, have also become more widespread throughout Illinois. Our ability to effectively manage these weeds without herbicides is somewhat questionable.
Secondly, great strides recently have been made in the effort to conserve soil. Part of this success has been due to the availability of herbicides for weed control. Once again, herbicides have directly substituted for tillage. Whether we can effectively control weeds in reduced and no-till systems without herbicides seems unlikely.
Altering planting date was also employed to alleviate weed problems. By planting corn or soybeans one to three weeks later than normal, we can stimulate weed seed germination early and destroy significant numbers of weeds with tillage prior to planting. This may work better with soybeans than corn. Long term corn studies have shown that the best time to plant corn in Illinois is around May l, with yields declining slowly between May 10 and 20, and greater reductions occurring after May 20 plantings (6).
Mulches and cover crops were often used to help conserve soil and provide weed suppression. Fall-seeded covers such as winter rye or hairy vetch compete with winter annual weeds and provide a mulch the following spring and help suppress summer annual weed emergence.
Cover crops must be killed prior to spring planting, which requires tillage or the use of a herbicide. U of I research, as well as research from other universities, suggests that a cover crop, such as winter rye, will suppress summer annual weed growth 50 to 80 percent (Gray et al., 1989).
A combination of cover crops with reduced herbicide inputs may allow for effective weed control as well as environmental protection. Additional research using cover crops and reduced herbicide inputs is still needed.
The environmental and health risks associated with herbicides becomes an important factor. Herbicides are the most common type of pesticide found in surface and groundwater. Currently, a number of herbicides appear on EPA's list of potential leachers and carry environmental hazard warnings on their registration label. Products with this warning have been found, generally in small amounts, in groundwater due to agricultural use.
In addition, although herbicides are typically less acutely toxic than insecticides, a number of commonly used corn and soybean herbicides are on EPA's list of potentially oncogenic (tumor forming) pesticides (1). Toxicity data for many of these products are incomplete, but the risk from drinking water or eating food contaminated with potential carcinogens is real enough to have concern.
The movement toward less intensive herbicide use is here. In my opinion, herbicides will and should remain a valuable tool for managing weeds. However, better and more discriminate use is possible and necessary.
Herbicides should no longer be thought of as a single solution. Weed management programs should emphasize crop rotation, competitive crop selection, mechanical weed control as appropriate to the crop and farm, weed scouting and identification and the use of economic thresholds where feasible.
Choice of herbicides and rates should be based on efficacy as well as associated health and environmental risks. Growers should consider alternating herbicides, selecting products that are less persistent, less likely to leach and controlling weeds postemergence.
There appears to be no clear-cut solution to the pesticide controversy. However, by adopting integrated pest management practices and using scientific principles to make pest management decisions, we can ensure the wise use of herbicides and a cleaner, healthier environment.
Table 1. Herbicide versus cultivation for annual weed control in corn (Liebl, 1989).
1. Anonymous. 1987. List of oncogenic compounds and suspects. U.S. Environmental Protection Agency.
2. Anonymous. 1935. The battle against weeds. International Harvester Co., Chicago, Illinois.
3. Gray, Susan G., William S. Curran, and Michael J. Mainz. 1989. Effect of cover crops and herbicides on weed control in corn and soybeans. NCWSS Res. Rep. 44., In Press.
4. Knake, E.L., M.D. McGlamery, and F.W. Slife. 1971. Controlling giant foxtail in Illinois. Univ. of Illinois College of Agric. Circ. 828.
5. Pike, D.R., 1988. Illinois Pesticide Use Survey. Univ. of Illinois, Urbana, Illinois.
6. Several Authors, 1989. Illinois Agronomy Handbook, Univ. of Illinois Circ. 1290.
7. Slife, F.W., 1973. Advances in weed control during the past 25 years. Crops
and Soils Magazine. March, pp. 5-7.
1William S. Curran is an Extension agronomist, Department of Agronomy, University of Illinois at Urbana-Champaign.
