From gopher@hafnhaf.micro.umn.edu Fri Feb 12 23:05:35 1993 Date: Fri, 12 Feb 1993 21:41:39 -0600 From: gopher@hafnhaf.micro.umn.edu (Gopher Login) To: london@sunsite.unc.edu Subject: Controlling Weeds in Sustainable... Doll, Jerry D. 1988. Controlling Weeds in Sustainable Agriculture. University of Wisconsin-Cooperative Extension Publication. pp. 1- 13. Controlling Weeds in Sustainable Agriculture J. D. Doll Can we grow crops without herbicides? Yes. We have been using chemicals to routinely control weeds only since the early 1960s. Previously, humans, draft animals, and machines did this work. Today, farmers are renewing interest in non-chemical alternatives for weed control. Some of this interest is due to environmental concerns. Intensive pesticide use has resulted in herbicide-resistant weeds and occasional groundwater contamination. In addition, failure to use proper precautions while applying herbicides can pose health risks. One means to protect the environment from the harmful effects of herbicides is to ban their use. Some farmers believe this will happen and want to be prepared to use alternative controls. Equally important for many growers is the need to reduce input costs when possible and practical by decreasing the amount of herbicides purchased. This publication discusses three alternatives to herbicides-cultural control, mechanical control, and biological control-and their effectiveness. Control Methods Cultural Control Cultural practices include commonsense farm management practices that have both direct and indirect effects on weeds. The goal of cultural weed control is to establish a vigorous crop that competes effectively with weeds. Cultural weed control practices include rotations of competitive crops and adapted, vigorous varieties; narrower row spacings and proper plant densities; proper seedbed preparation; adequate fertilization; and appropriate insect and disease control. These practices favor more vigorous crop growth and development, and tend to be based on long-term strategies rather than the short-term effects of herbicides. For instance, rotating from corn to oats and/or alfalfa almost completely eliminates wild proso millet, one of the state's most serious new weeds that is particularly competitive and hard to control in corn. Alfalfa and small grains control the weed because they become established before the soil is warm enough for wild proso millet to germinate. Winter wheat, early planted peas, and spring barley also effectively displace wild proso millet. Rotating from annual row crops to alfalfa helps manage weeds in other ways, too. Seed longevity of annual grosses like giant foxtail and barnyardgrass is relatively short. If these grasses do not produce new seeds while fields are in alfalfa for 4 or 5 years, the seed reservoir is reduced significantly. However, this approach has little effect on weeds such as velvetleaf, wild mustard, pigweed, and lambsquarter, which have very long-lived seeds. Thus, crop selection decisions are based on knowing the particular weed problems for each field. Plant the most competitive crops in the weediest fields and the least competitive ones in the cleanest fields. You should also select vigorous varieties adapted to your farm's growing conditions and resistant to as many pests as possible. Consider placing heavily infested fields in the long-term set-aside program (CRP acres). The permanent cover should prevent buildup of annual weeds and allow the soil seed bank to diminish. However, pay special attention to prevent perennial weeds from spreading in setaside acres. Proper row spacing and plant density assure rapid coverage of the soil by the crop, while still allowing row cultivation and other field operations. For example, herbicides seldom are required in small grains because their narrow row spacing, high plant density, and early rapid growth leave little opportunity for weed establishment. In addition, a well-prepared seedbed and adequately fertilized soil assure rapid, uniform germination and good crop growth. This enhances the crop's competitive ability. Likewise, healthy crops maintain a high degree of competitiveness against weeds, so keep crops free of insect and disease damage. Mechanical Control Mechanically controlling weeds is another alternative to herbicide use. Primary and secondary tillage tools, the rotary hoe and row crop cultivator, are customary implements for a mechanical weed control program. The measures used to prepare a crop seedbed are part of a mechanical weed control program. In fact, no-tillage systems require herbicide use to assure a relatively weed-free environment in the absence of soil disturbance. On the other hand, moldboard plowing kills all annual weeds and perennials having a simple taproot system (dandelion, yellow rocket, alfalfa, white cockle, etc.). But perennials with rhizomes (quackgrass) or creeping vegetative roots (Canada thistle, bindweeds, hemp dogbane, milkweed) will recover after tillage. Rotary hoeing is sometimes referred to as "blind tillage". When done on a timely basis, the weeds are between the germination and emergence stages, and you do not see most of the seedlings you are destroying. In fact, weed seedlings more than 1/2-1 inch tall are not easily controlled by the rotary hoe. Rotary hoeing is limited to large-seeded crops like corn and soybeans that escape injury because they are planted 1-2 inches deep and have a rapidly developing root system. Similarly, large-seeded weeds like velvetleaf and shattercane will not be adequately killed by this practice. Begin rotary hoeing 7-10 days after planting. Best results are obtained when soils are relatively dry because the rotary hoe may just transplant the weeds if soils are moist. Drive at least 6 mph in the same direction as the crop rows. Try to match the width of the rotary hoe to a multiple of the number of rows of your planter. For example, use an eight-row rotary hoe if you have a four-row planter, so you can drive in the planter wheel tracks as you rotary hoe. This minimizes soil compaction. Some growers rotary hoe their fields several times at 5- to 8-day intervals. A row cultivator should be used once soybeans are more than 2 inches tall and corn is more than 3 inches tall. Adjust the cultivator sweeps or teeth to dislodge or cover as many weed seedlings as possible. Three-point mounted cultivators should have the front and rear sweeps at approximately the some depth. Seedling weeds can be killed by cultivating 1-2 inches deep. Larger weeds will require deeper cultivation. Be careful not to damage the crop root system, especially when cultivating crops 10 inches or more in height. And be sure you are killing all weeds between rows. As when using a rotary hoe, soils should be relatively dry when using a row cultivator. A cultivator should roll dry soil into the crop row to cover small weeds. But when soils are too moist, the weeds may only be transplanted. Driving speed will usually vary between 3-6 mph. Adjust your cultivator speed to the soil condi- tions and crop height to avoid covering crop plants with soil. You should slow down where plants are smaller. "Cultivator blight" is a term often used to describe crop injury resulting from poor cultivation. Common causes of cultivator blight are driving too fast, poor equipment adjustment, and operator boredom. Take a break periodically to reduce boredom and increase alertness. Machinery manufacturers have made significant improvements in cultivators in the last 20 years. Many cultivators now have mechanisms to prevent covering small crop plants with soil. Heavy duty cultivators also are available that provide effective weed control in the high crop residue, more compacted soils common to reduced tillage systems. Remember, though, herbicides may still be needed for initial weed control in no-till systems. The approximate costs of several mechanical weed control measures are given in the table on this page. These values are guidelines only; your actual costs may be different. On the overage, for a corn crop that is rotary hoed twice and cultivated twice, it would cost about $12.75/a for these practices. Table 1. Approximate costs of several mechanical weed control practices" Input costs Total Practice Tractor Implement Labor cost - - - - - - - $/Acre - - - - - - - - - - - - - - - - MB plow (4-16 in) 4.55 4.19 1.86 10.60 Chisel plow (15 ft) 2.53 .86 .66 4.05 Tandem disk (16 ft) 1.36 1.23 .56 3.15 Field cultivator(18 ft) 1.66 .69 .50 2.85 Row cultivator (4 rows/36-inch rows) 1.29 .99 .95 3.23 Row cultivator (6 rows/30-inch rows) 1.47 .90 .76 3.13 Ridge cultivator (4 rows/36-inch rows) 2.27 1.71 .96 4.94 Ridge cultivator (6 rows/30-inch rows) 2.50 1.87 .76 5.13 Rotary hoe (16 ft) .59 .88 .39 1.86 Adapted from Minnesota Farm Machinery Economic Cost Estimates for 1987, Univ. of Minn. Ext. Buil. AG-FC-2308, 1987, Limitations to Mechanical Control. There are several limitations to effective mechanical weed control, including wet weather, large acreages, problem weeds, and other demands on operator time. For example, because mechanical weed control requires a relatively dry soil, a 1- or 2-week rainy period as crops emerge may eliminate rotary hoeing and delay cultivation so that serious weed competition occurs. Under these conditions, you may need to use an herbicide to suppress or kill weeds until you can cultivate. Some operators have several hundred acres of cash groin crops. To depend solely on mechanical control, you must have enough rotary hoes, cultivators, and tractors of the right size, as well as enough operators, to do the job rapidly and on a timely basis. Growers with smaller acreages may not have the time to do the mechanical practices when needed if they have livestock to care for, hay to harvest, or an off-farm job. Several weed species are difficult to control unless her- bicides and mechanical measures are used. These include wild proso millet and woolly cupgrass in corn; and Canada thistle, hemp dogbane, bindweeds, and quackgrass in corn and soybeans. Newly introduced weeds often show up in scattered patches or along the headlands and field borders. These are often best controlled or eradicated with herbicides before large areas are infested. Biological Control In the strict sense of biological control, there are cur- rently no means of using insects or diseases to control serious weed pests at the form level. Biological control programs are costly to initiate and research, and are almost always done on a multi-county, state, or regional basis. In Wisconsin, we are exploring the use of a seedeating weevil to suppress musk thistle in the southeastern part of the state. However, the project has been suspended by state agencies because this insect also is capable of attacking the dune thistle, a threatened plant on the sandy shores of Lake Michigan. Every control strategy has its own set of risks. One requirement of biological control is that the attacking organism must be very specific in what it affects. The feeding habits of the musk thistle weevil are not so specific to prevent it from affecting the dune thistle but, unfortunately, are specific enough so as not to seriously disrupt the life cycle of plumeless thistle, one of our serious posture weeds in southwestern and south central Wisconsin. One common cropping practice in Wisconsin is actually a form of biological weed control-companion crop alfalfa establishment. The early planted and fast-emerging oats or barley displace (or "replace") common annual weeds like lambsquarter, pigweed, rag- weed, smartweed, foxtail, and barnyardgrass that might otherwise germinate. This explains why herbicides seldom are used in companion crop seedings but usually are used to help establish and improve forage quality when alfalfa is direct seeded. Cover crops also can be considered a form of biological weed management. This practice should be used on all set-aside acres to prevent most weeds from producing seed, save soil, and provide wildlife habitat. Another example of biological control is to grow one plant that prevents or suppresses the development of other plants. This is called "allelopathy," which means 'mutually harmful." It is the opposite of o symbiotic relationship. Allelopathy is an area of active research. The most promising system developed to date is based on winter rye. The rye is planted in late summer or early fall. The crop is killed in the spring with Roundup or Gramoxone, and corn, soybeans, or other crops then are no-till planted into the dead or dying rye. If the rye is killed by plowing or disking, its inhibitory effect on weeds is lost. Some varieties are more effective than others; Wheeler rye appears to be one of the most active. Spring oats also can be planted in late summer to use as next season's allelopathic crop. This crop is less effective than rye but is winterkilled, so an herbicide is not needed the following season. These allelopothic systems are still being tested and should only be tried on a small scale to see how they perform under Wisconsin growing conditions. Be prepared to use a no-till cultivator or postemergence herbicides if needed.