Growing a competitive crop – first step in weed control

B. Frick, E. Johnson - Scott Research Farm

Problem
Crops inevitably grow in relationship with weeds. Crops and weeds compete with each other for limited resources, such as nutrients, light, water, space. How can this competitive relationship be managed to give the advantage to the crop?

Background
Competitive ability can be viewed two different ways: ability to tolerate competition (maintain yield in the presence of weeds) and ability to suppress weeds. Studies indicate that the competitive aspects of tolerance and suppression may be correlated. Factors that increase competitive ability include rapid germination, early emergence, seedling vigour, rapid leaf expansion, number of stomates, rapid canopy development, plant height, early root growth, and extensive root systems.

Crops differ in competitive ability with weeds. In general, barley is more competitive than spring rye. Both are more competitive than wheat or oat, and flax is less competitive. Durum wheats are less competitive than spring or winter wheat. Wheat is considered more competitive than pea, and then in order of decreasing competitive ability, pea, potato, soybean, flax, and bean. Most pulse crops, like lentil, are poor competitors. Canola offers poor competition to weeds in the seedling stage, but can compete well once it becomes established.

Fall sown crops such as winter wheat and fall rye offer excellent early season competition, and do not require spring cultivation. These crops are especially effective at competing with spring germinating annual weeds such as wild oat or green foxtail. Fall sown crops also allow partial fallow after harvest, for further weed control. Greenfeed or silaging annual grain crops can be used as a partial fallow replacement. Two years of harvesting barley for silage at an early stage (heads fully emerged) reduced wild oat densities to levels similar to wild oat herbicide applications.

Perennial crops such as crested wheatgrass, brome, sweet clover and alfalfa can be very competitive with annual weeds by eliminating tillage’s stimulatory effect on annual weed seeds. Perennial crops can also offer competition against perennial weeds that lasts beyond the annual crop season. Crested wheatgrass is more competitive than other forages.

Each crop has many different cultivars or cultivated varieties. Several major crops have trait variability that affect competitive ability. In the past, crop breeding programs placed relatively little emphasis on developing superior cultivars for growth under weedy conditions, but this is changing.

A test of 250 wheat varieties in Australia showed that old standard varieties (those released between 1880 and 1950) suppressed weeds more than most of the current varieties. Strongly competitive genotypes had high early biomass accumulation, large numbers of tillers, and were tall with extensive leaf display. Yield differences in weedy conditions were not found when herbicides were used. Taller cultivars had fewer weeds than shorter cultivars. Cultivars also differed in the dormancy of wheat seeds and thus, in the number of volunteer wheat plants in subsequent years.

In a study of eight wheat cultivars at Scott and Saskatoon, CDC Merlin, AC Minto and Columbus were found most competitive, and Genesis and Oslo least competitive with weeds. Spring spelt was the most competitive wheat in tests with model weeds (crop plants used to simulate weeds). Research indicates that longvined, rapidly developing pea varieties were more competitive than shorter vined cultivars. Leaf type might be expected to make a difference, but studies at Morden did not find an advantage to leafier varieties competing with wild mustard. Pea cultivar (tall, leafy Century; tall semileafless Tipu; short leafy Express) had no effect on grassy weed populations.

Semidwarf winter wheat varieties resulted in a 14-30% greater yield reduction from downy brome (Bromus tectorum) than did taller cultivars. Winter wheat was more effective in suppressing quack grass than tall or semidwarf spring wheats. Research is currently underway at the University of Saskatchewan to develop oat cultivars with more weed competitiveness. Recently developed leafy forage oat genotypes have been found to be more competitive with wild oat than conventional milling or feed oat genotypes.

Differences among cultivars depend on the entire cropping environment, not just the presence of weeds. For instance, in years with average or below average moisture, a semi-leafless pea cultivar seeded at reduced rates lost more to competition with wild mustard than did a leafy cultivar, and the semileafless cultivar lost more to competition when it was seeded at low rates than when it was seeded at high rates. Under drought conditions, wild mustard interference caused greater yield reductions in the leafy cultivar compared to the semi-leafless cultivar.


Conclusions
Competitive ability varies among crops and among cultivars. Crops generally are most competitive if they have vigorous growth, especially at stages when weeds are emerging. Competitive ability is determined by characteristics of both crop and weed, and also by their environment.


Funding
Provided by the Canada-Saskatchewan Agri-Food Innovation Fund


Contact information
Brenda Frick, Ph.D., P.Ag.
Prairie Coordinator
Organic Agriculture Centre of Canada
c/o Department of Plant Sciences
University of Saskatchewan
51 Campus Drive, Saskatoon
Saskatchewan, Canada S7N 5A8
Tel: (306) 966-4975
Fax: (306) 966-5015
Email: brenda.frick@usask.ca


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