Weeds

We commonly think of weeds as particular plant species such as wild oats, Canada thistle, or dandelions. A more general definition of a weed is a plant that is growing where someone does not want it to grow; a more optimistic definition of a weed is that it is a plant whose virtues have yet to be discovered. There are about 120 weed species infesting major field crops throughout the province, the vast majority of which are not native. Most originated in Europe and Eurasia, and were introduced into Saskatchewan by explorers and early settlers. Some arrived as contaminants in livestock feed, in packing material, or in grain to be used for planting the first crops. Others were introduced as ornamentals and have escaped cultivation to become serious weed problems. Of the forty-one weed species that have been declared noxious under the province's Noxious Weeds Act, thirty-nine are not native to the province.

Weeds are one of the major constraints to profitable crop production. It is commonly accepted that more time, energy and resources go into fighting weeds than in any other agricultural activity. Weeds cause major economic losses because they compete with crop plants for water, nutrients and light, and thus reduce crop yields. Crop yield reductions vary depending on a number of factors including: the weed species present (some are more competitive than others); the crop the weeds are growing in (some crops are more competitive than others); the relative time of emergence of the weeds and the crop (weeds that emerge before the crop cause much greater losses than weeds that emerge after the crop is up); the weather conditions (some weeds do best in hot, dry conditions, others thrive in cooler, wetter conditions); and crop management practices such as seeding dates and rates, seeding depth, fertilizer amounts and placement, crop plant density and crop row spacing. Farmers can minimize crop losses due to weeds by adopting appropriate crop management practices and crop rotations. However, in spite of farmers' best efforts and the fact that they spend several hundred millions of dollars on herbicides each year, it is estimated that crop yield losses in Saskatchewan still amount to more than a half billion dollars annually.

While reductions in crop yield represent the major economic loss due to weeds, they can cause losses in other ways, interfere with human activities, and pose a danger to health. Some weed species can affect the grade, and thus the value, of crop and livestock products (absinthe in wheat, stinkweed-tainted milk). Some weeds are toxic to livestock and may result in death or reduced productivity. Sharp awns and burs can cause physical injury and discomfort to livestock, and can also lead to reduced production or product quality. The presence of green weeds in a mature crop can delay crop harvest, and this may result in losses from weathering or sprouting. Green weed material that contaminates grain can lead to heating and spoilage of the grain in storage. Weeds also cause problems by reducing water flows in irrigation canals and drainage ditches, and by increasing snow-plowing costs on highways and secondary roads. Some weeds contribute to the discomfort endured by hay-fever sufferers and asthmatics; others such as poison ivy and stinging nettles are severe skin irritants; and some such as poison hemlock are extremely toxic to humans.

A wide variety of control measures which have been developed to aid in the battle against weeds can be categorized as follows: prevention aims to avoid the introduction of new weed species into an area or to prevent the spread of existing species to uninfested areas; cultural controls include crop rotations and crop management practices that aim to maximize the competitiveness of crops against weeds. Examples include the use of high-quality seed, optimum seeding rate, date and depth, optimum fertilizer rates, and placement and appropriate row spacing. Mechanical (physical) controls include the use of mulches, timely mowing, tillage, and burning to prevent or destroy weed growth. Chemical control involves the use of non-selective and selective herbicides to control weed growth. After the introduction of 2,4-D to Saskatchewan in 1946, selective herbicides quickly became the preferred means of weed control on most farms, and they still hold that position. The recent, widespread adoption of zero-tillage/direct-seeding crop production systems is largely due to the availability and relatively low price of the non-selective herbicide, glyphosate, and a wide variety of selective herbicides that control a broad spectrum of weeds in crops. Biological control utilizes a living organism (insects, diseases, and grazing animals) to control weeds. Successful control of some weeds in Saskatchewan has been achieved with insects, but development of biological controls is a slow, difficult process. Integrated weed management refers to the practice of integrating several control measures.

It is unlikely that weeds can be entirely eliminated. Weedy plant species have evolved along with agriculture for the past 10,000 years and have repeatedly demonstrated their resilience and ability to adapt to all of the management practices, and to control the measures farmers and weed scientists have been able to devise. Some species have adapted well enough to survive in highly disturbed areas and so thrive where soils are intensively tilled. Others are best adapted to undisturbed sites, and become more problematical in fields that are under zero-till or direct-seeding management systems. Many weedy species are capable of producing large quantities of seed, and have evolved very effective seed dispersal mechanisms that involve wind, water, animals, birds, and humans. Many have developed varying degrees of seed dormancy that constitute a very effective survival mechanism. Some weeds, especially perennial species, can propagate and spread by both seed dispersal and vegetative means. More recently, some weed species have evolved resistance to herbicides: this continues to be an increasing problem in Saskatchewan and throughout the world, wherever crop production involves the intensive use of herbicides. As a result, more attention is being paid to the development and adoption of integrated weed management strategies that aim to reduce our reliance on chemical herbicides. However, weeds are extremely adaptable and have shown that they can survive in virtually every agricultural situation. Thus the farmer's goal is not to eradicate weeds, but to adopt economical and environmentally sustainable weed management practices that minimize their impact and spread.

F.A. (Rick) Holm