From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Fall 1997, No. 5


It doesn't take many weeds in a grain crop to reduce yields. Weeds decrease crop yield because they compete with the crop for water, light and nutrients. We can't do much about the water and light, but we can manage the nutrients to give the crop a competitive edge.

Weeds respond to nutrients like most crops, but they can also be more responsive and more adaptable. Weeds show similar growth patterns and have similar nutrient requirements as crops. Perennials can easily get ahead of annual crops because they have an early and greedy appetite for nutrients due to their already well developed root systems. Aggressive weeds, like lambsquarters or wild oats, have a faster developing and more extensive root system than crops. Many weeds have a special ability to utilize high nutrient levels by luxuriant growth; others show the ability to grow better on soils with low levels of nutrients.

As a generalization, weeds are better able to withstand adverse environmental conditions, including nutrient stress, than crops. Weeds have a competitive advantage under such conditions.

Fertilization can be used as a weed management tool, especially if it's adopted with an integrated pest management approach. The key is to fertilize the crop and not the weed --- placement is critical. Alberta researchers have shown that banding nitrogen can have dramatic effects on weed populations and biomass production. In zero-till barley they found banding nitrogen, at increasing rates, decreased green foxtail populations by more than 95 percent and stinkweed populations by 80 percent. In wheat, nitrogen increased both wheat seed yield and foxtail barley biomass. However, banding the nitrogen produced less foxtail barley and more wheat yield than broadcasting the nitrogen.

Fertilizer placement for weed suppression is just as critical for immobile nutrients like phosphorus. Alberta studies have shown that phosphorus placement can have a pronounced effect on the competitive ability of wild oats in barley. In one experiment, seed-placed phosphorus decreased wild oat production by more than 50 percent compared to broadcast phosphorus applications. When more available to the barley, phosphorus enhanced its growth, thus preventing wild oat seed production. However, when broadcast application increased phosphorus availability to the wild oats, their production increased at the expense of the barley.

Phosphorus helps roots and seedlings develop more rapidly, getting the crop off to a good start. A fast start is essential to make the crop competitive, especially against aggressive weeds. Canadian researchers have reported that each day of emergence of wild oats before wheat or barley increased yield losses by about three percent. And, yield losses declined by the same amount for each day wild oats emerged after the crop. Any practice that will encourage the crop to emerge before weeds will likely increase yield.

Put your fertilizer where it will do the most good --- next to the crop. Don't give weeds any extra help, they don't need it!

--- TLR ---

For more information, contact Dr. Terry L. Roberts, Western Canada Director, PPI, Suite 704, CN Tower, Midtown Plaza, Saskatoon, Saskatchewan, Canada S7K 1J5. Phone (306) 652-3535. E-mail:
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