The Control of Weeds in Seed Crops

W. C. SHAW AND L. L. DANIELSON.

THE SEQUENCE of events that occurs when cultivated fields are abandoned is well known. Without the efforts of man, an abandoned cultivated field eventually will be occupied by the climax vegetation characteristic of the geographical area.

The fundamentals of plant ecology emphasize that plant successions always occur in the direction of the climax vegetation rather than toward the growth of more economic crop plants. Weeds such as crabgrass, pig-weed, ragweed, and lambsquarters represent the first step in the plant succession away from economic crops toward the climax vegetation.

In our efforts to produce economic crops, we attempt to utilize fully all available technology to stabilize and balance the vegetation at a highly productive level and to prevent the occurrence of plant successions to the climax vegetation. We attempt to delineate and eliminate or correct the limiting factors in the production of economic crops.

One of the most important places to start is with the control of the weeds, which compete directly for all production factors and most critically for those that are limiting.

Seeds of weeds often are disseminated in crop seeds. The use of weed-free crop seeds is a sound starting place for an effective program to control weeds. Regardless of the effectiveness of weed-control practices, weed-control programs are relatively ineffective unless the program is accompanied by efficient weed-control practices in crops grown for seed production and weed-free seeds are available to farmers.

Weeds are among the greatest contributors to production costs on American farms. Losses due to weeds have reached an estimated 4 billion dollars annually.

Weeds compete with crops for water, light, and mineral elements. One plant of common mustard (Brassica kaber) requires twice as much nitrogen, twice as much phosphorus, four times as much potassium, and four times as much water as a well-developed oat plant. Ragweed (Ambrosia artemisiifolia) requires three times as much water as corn to produce a pound of dry matter.

The reduction in yields of seed crops and quality caused by weeds is influenced by the composition and the density of the weed population, the length of time the weeds are allowed to grow in competition with crop plants, and soil and environment.

Numerous investigations on weed-crop competition in the production of soybeans, corn, rice, small grains, vegetables, forage plants, and other crops grown for seeds indicate that weed competition is most serious during the first 30 days after they emerge. The emergence of weeds should be prevented, or they should be controlled immediately after emergence for maximum reduction of competition.

The competitive effects of barnyard-grass (Echinochloa crusgalli), a serious weed in ricefields, are illustrated by results from studies in Arkansas in which average yields obtained from heavily infested rice sprayed with the herbicide, isopropyl N-(3-chlorophenyl) carbamate [CIPC] at 4, 6, 8, and 10 pounds per acre were 85, 90, 94, and 96 bushels per acre, respectively. The yield of rice on untreated check plots averaged 56 bushels per acre.

WEED SEEDS are disseminated by wind, water, animals, and people. Many weed seeds have modifications and adaptations for dissemination by one or more methods. For example,the seed of Canada thistle resembles a parachute and can travel long distances through the air. Many weed seeds will float for indefinite periods and may be carried long distances by water. Irrigation canals and ditches are important means of disseminating weed seeds in irrigated farming areas.

Wild and domestic animals assist in the dissemination of weeds. The seeds of many weeds pass through the digestive tract of animals without impairment of viability.

Weeds may be disseminated by mass means of modern transportation. Weed seeds are often transported in the packing about trees; in soils and of gravel used in construction; in refuse or mud; on the wheels of vehicles; in soil adhering to plows, cultivators, and other farm tillage implements; in threshing machines, harvesting combines, hay balers, and portable seed cleaners; and on tires of automobiles and airplanes.

The prevention of the introduction and spread of weeds and weed seeds is important. Preventive methods include the use of weed-free crop seeds.

The viability of weed seeds in screenings of grain or hay can be destroyed by grinding or ensiling before they are fed to livestock. The viability of weed seeds in manure should be destroyed by thorough fermentation before it is used on farms. Livestock should not be permitted to move from weed-infested areas directly to uninfested areas. Grain drills, harvesters, cleaners, hay balers, and other farm machinery should be cleaned before moving from infested areas. The use of gravel, sand, and soil from areas known to be heavily infested with weeds should be avoided. Nursery stock should be inspected for the presence of weed seeds,, tubers, and rhizomes of perennial weeds. The banks of irrigation ditches, fence corners, fence lots, roadsides, railroad rights-of-way, and other uncropped areas should be kept free from weeds.

THE METHODS of control include mechanical techniques, such as hand hoeing, tillage, mowing, flooding, burning, and smothering; cultural methods, such as crop rotation and crop competition techniques; biological methods, which involve the use of insects, plant diseases, and other organisms to control weeds; and chemical methods, including the use of selective and nonselective herbicides as foliage and soil applications. A well-balanced program of weed control should make maximum effective use of hand, mechanical, tillage, cultural, biological, and chemical methods.

The relationship of plant ecology to control of weeds.

Increased emphasis has been given the use of herbicides. We are now entering an era in agricultural production in which various forms of energy are used to control weeds and to control insects and diseases and for crop protection in many ways.

The chemical energy of herbicide molecules is a reservoir out of which must come our most efficient weed-control practices for the future. There is increasing evidence in weed-control techniques that mechanical equipment will be used more and more in the future to transport chemical energy. The herbicides will be the sources of energy and perform the work rather than the machine with which they are applied.

This does not imply that the need for mechanical equipment will be reduced. It means greater emphasis on the specificity and accuracy of equipment in placing the chemical energy in the exact amount, in the exact place, and at the exact time for the chemical to perform its maximum work.

CHEMICAL methods include the use of herbicides as soil-incorporated preplanting, preemergence, and post-emergence treatments; directed post-emergence treatments; band applications immediately over the row; and soil sterilization.

Herbicides offer almost unlimited opportunities in versatility and completeness in the control of weeds.

To use herbicides most effectively, one should know their nature and properties, sites and mechanisms of action, metabolic fate in plants and soils, and the effects of environment on their performance.

Herbicides may be classified on the basis of their mode of action and use. The selectivity of herbicides is relative and depends largely on the rate of application and the stage of growth of plants.

The number of different types available for use on farms makes it possible to control a wide variety of specific weeds and a broad spectrum of weed populations in the various weed-crop situations.

Herbicides kill weeds by inhibiting essential processes in plant growth, such as respiration, photosynthesis, transpiration, and mitosis (an important process in cell division) or by preventing the biosynthesis of elements essential to the growth of plants.

2,2-Dichloropropionic acid [dalapon], a selective, systemic herbicide used for the control of grasses, inhibits the enzymatic synthesis of pantothenic acid, a vitamin B essential to the growth of plants. When dalapon inhibits the biosynthesis of this essential metabolite in treated plants, they are seriously damaged or killed.

Some herbicides may kill plants by interfering with mitosis and other reproductive processes in plants. Compounds such as CIPC cause abnormal chromosome behavior in plants.

Herbicides may also kill plants by preventing the normal function of essential processes in plant growth. For example, 2-chloro-4,6-bis (ethylamino)- s-triazine [simazine], a new herbicide very effective as a preemergence treatment for the control of weeds in corn, kills plants by inhibiting the efficiency of the chloroplasts to function in the process of photosynthesis. Treated plants are thus inefficient in the production of sugar and starches, which are essential to their growth.