Crops contamination with weeds

Just before Thanksgiving the government announced that it had destroyed cranberries contaminated with a chemical, aminotriazole, which produced cancer in rats. The cranberries were from a lot frozen from two years earlier when the chemical was still an approved weed killer. The animal studies were not completed until 1959. Even though there was no evidence that the 1959 crop was contaminated, cranberry sales dropped precipitously and public fears about dangerous chemicals in food lingered. 

Dealing with such problems and adopting methodologies to reduce any risk of crop contamination places additional labour, infrastructure and financial burdens on growers. To address such concerns it has been proposed that non-food crop plants unrelated to current food crops and native flora (to avoid risk of crosspollination) should be used as potential hosts for engineered industrial use traits. Crambe, (Crambe abyssinica) has been identified as a suitable model oil crop plant (EPOBIO 2007). Crambe is a plant that has already been commercialised on a relatively small scale to exploit its high erucic acid content. Elsewhere, safflower has been proposed as a potential candidate, as well as the use of algae, moss and the aquatic plant duck weed in contained bioreactor systems. 

Recently, in Germany it was found that salads (especially ready-packed rocket salads (Eruca saliva), sold in supermarkets) and salad mixtures can be contaminated with PA containing plants, mainly Senecio vulgaris, a typical weed of field crops [38]. 

There are many reasons that weeds need to be controlled. The direct competition that leads to yield loss is the most obvious. Others include the contamination of the crop with seeds from aggressive or poisonous... 

Often, the rates of fertilization in intensively managed agriculture are intended to satiate the needs of crop plants for these chemicals, so their productivity will not be limited by nutrient availability. However, excessive rates of fertilization have important environmental costs. These include the contamination of ground water with nitrate eutrophication of surface waters caused by nutrient inputs (especially phosphate) acidification of soil because of the nitrification of ammonium to nitrate large emissions of nitrous oxide and other nitrogen gases to the atmosphere, with implications for acid rain and Earth s greenhouse effect and the need to use herbicides to control the weeds that flourish under artificially nutrient-rich conditions. 

Contact reactions to plants in farm workers cover a wide spectrum. Reactions occur to poison ivy, oak and sumac [145] which are often not reported. Exposure can occur when clearing fence rows and sometimes when handling domestic animals. These plants are not usually found with the crops, where herbicides are used for weed control, but are more often seen on fence rows. Sometimes contaminated hair of farm animals is the source. The details of Toxicodendron dermatitis were given in the previous section. 

For herbicides with low vapor pressures, the major mechanism of transport within the soil is movement in the soil water. The ability of a soil-acting herbicide to provide effective weed control depends upon its ability to move to its target site within the soil profile. Some selectivity between weeds and crops can be achieved by preparation of molecules with appropriate penetration depth within the soil. For the shallow-rooted weeds, the herbicide only needs to move to approximately the top 5 cm of the soil profile further movement takes the chemical beyond its target site and increases the potential for environmental contamination. An understanding of water movement in soils is therefore fundamental in determining the transport of herbicides to their target sites. 

The two most used herbicides in this class are 2-methyl-4-chlorophen-oxyacetic acid (4.55) (MCPA or Methoxone )> and 2,4-dichlorophen-oxyacetic acid (2,4-D, or Chloroxone ). The use of 2,4,5-trichlorophen-oxyacetic acid has been suspended, because the usual process of manufacture contaminates it with a teratogenic dioxin (6.51). Tests carried out on 30 annual weeds that normally impoverish cereal crops showed that, by choosing the right compound and applying it at the right time, almost all the weeds can be killed. It is not known why cereals are relatively unaffected. In some experiments cereals were made to absorb as much of the phenoxyacetic acids as the weeds do (normally they absorb less), but the cereals remained unharmed (Wood, Wolfe and Irving, 1947). On the whole, dicotyledons are killed and monocotyledons survive, but there are striking exceptions. Thus onions (monocotyledons) are susceptible to the phenoxyacetic acids, whereas chickweed and cleavers (dicotyledons) are resistant because they have an enzyme that removes the side-chain. 

---