Corn & The Environment

Corn & Pesticides

The quantity of pesticides used per hectare of land planted to corn is similar to that of major spring-seeded crops (Ontario Ministry of Agriculture, Food and Rural Affairs, Publication 75, Guide to Weed Control). Per tonne of crop yield, the rate of usage tends to be lower with corn. Typically, about 2-3 kg/ha of active ingredient are applied. Most of this involves herbicides applied just before or after the time of spring seeding.

The quantity of pesticide usage per hectare and the real expenditure for pesticide purchases is trending downward for corn and other major field crops in Ontario. (See tables below.) Pesticide usage in Ontario agriculture declined by more than 28% between the years 1983 and 1993 according to survey data collected by the Ontario Ministry of Agriculture, Food and Rural Affairs (1984, 1989 and 1994). Many anticipate that this trend will continue with the expected registration of newer, safer pesticides which are effective at very low rates of application. This trend to reduced rates is occurring despite a reduction in the amount of soil tillage. (Soil tillage has traditionally been used as a non-chemical method of weed control.)

Public concerns with respect to pesticide usage in agriculture are two-fold: the risk of food contamination, and the risk of contamination of water, soil and air.

Concerning the former, surveys (for example, Agriculture Canada and Agri-Food Canada, 1989) have rarely found detectable pesticide residues in grain corn or food products made from grain corn. In the rare occasion when a pesticide residue is detected, the concentration is usually below Health Canada maximum residue level standards. As with other food products, health risks - to the extent to which these occur - are almost exclusively the result of natural compounds and biological contaminants (for example, bacterial contaminants and mycotoxins) (Ames et al., 1987).

Atrazine

Surveys of river and stream water quality in farming areas of southern Ontario have found that atrazine, a traditional corn herbicide, can be present, especially during the spring "run-off" season. In an effort to reduce or eliminate such contamination, the Health Canada label requirement for atrazine, which dictates how this product can be legally used in Canada, was changed in 1991 to prohibit autumn usage (the source of most of the atrazine found in early spring run-off water) and reduce maximum permitted application rates in spring. This change occurred as a result of a request to government made by jointly by atrazine manufacturers and Ontario farm organizations.

Usage of atrazine has declined substantially in Ontario (See table below), in part because of concerns over surface water quality, but mainly because this long-persistent herbicide is not well suited to the crop rotations which are now used by most farmers. Low levels (1 kg/ha of active ingredient or less) of atrazine are commonly used by many farmers, in combination with other newer herbicides, as a low-cost means of controlling potentially troublesome weed pests. This is in contrast to application rates of 3-5 kg/ha which were common 10-20 years ago.

It is expected that these changes in use patterns will mean a major reduction in the incidence of atrazine detection in ground water and off-farm surface water.

Toxicological studies indicate that atrazine does not constitutes a significant health hazard. The "LD50" (level of intake which has a 50:50 chance of causing death) is about 3080 mg per kg of body weight for atrazine as compared to values as low as 1 mg/kg for some pesticides, and values of 1000 mg/kg for aspirin and 127 for caffeine. Because of the low toxicity, containers of atrazine are not required to list any hazard warnings, unlike most pesticides and many household products. Care must be taken that declining use of atrazine does not mean increased usage of other pest control products which are more toxic.

Surveys have shown relatively few cases of river or ground water contamination by corn pesticides other than atrazine. With the expanding usage of conservation tillage (which reduces surface water run-off), the registration of newer, more environmentally benign products for pest- control usage in corn, and reduced overall pesticide usage, the amount of contamination is expected to become even lower.

Insecticides

Although usage of soil-applied insecticides increased rapidly during the 1970s and early 1980s because of the across-Ontario migration of the corn rootworm insect, such usage has now decreased because of the wide-spread use of crop rotations. (Corn rootworms only cause damage when corn is grown in the same field for two successive seasons.) New integrated pest management methods, which will permit farmers to better predict when usage of insecticides is needed to prevent economically important crop damage where corn must be grown after corn, will mean a further reduction in soil insecticide usage.

Insecticides and fungicides are rarely applied to corn after planting, thanks to the efforts of cornbreeders and the resulting near absence of leaf-destroying injury by insects or diseases.

Biotechnology offers the potential, by making corn resistant to insects such as European corn borer and corn rootworm, to eliminate virtually all pesticide usage in corn production in Canada. (See section on Corn breeding and genetics .)

Efforts to reduce pesticide usage in corn production are having an effect. A major survey of 1192 farm wells and ground water bores in late 1991 and early 1992, in corn-growing areas of Ontario (Ontario Farm Groundwater Quality Survey, Agriculture and Agri-Food Canada, 1992 and 1993) found only two sites where pesticide concentrations exceeded Health Canada safe drinking water standards. This should be compared to survey results showing that 31% (autumn survey) and 36% (spring survey) of sampled wells exceeded provincial maximum acceptable standards for coliform bacteria.

Thanks to a regulation under the Ontario Pesticides Act which was instituted as a result of a request from AGCare (Agricultural Groups Concerned About Resources and the Environment - a consortium of Ontario farm groups), all users of farm pesticides must be certified. The certification process includes a training program and a written examination.