ONTARIO GRAIN FARMER AGRONOMY 27 caused a 3.5-fold greater corn yield loss than black nightshade at the exact same density (10 weeds per square metre).” For most producers, the impact of broadleaf weeds may not be a surprise, but factoring in the density had a sobering effect. With lamb’s quarters, an increase from one plant per square metre to 25 caused a 4.7-fold increase in yield loss (from 12 per cent to 56 per cent). Triazine-resistant biotypes caused a four per cent decrease in corn yield due to a “fitness penalty” and may be easier to control with an alternative herbicide. Of the glyphosate-resistant weed species, waterhemp had the lowest yield loss (19 per cent) compared to Canada fleabane (54 per cent) and giant ragweed (72 per cent). “Waterhemp is a heliotype that responds to high light intensity and corn just outgrows waterhemp,” said Sikkema. “Canada fleabane grows above the crop, and the same with giant ragweed. In summary, annual broadleaf weeds result in greater corn yield loss than annual grasses.” MONEY TALKS Another timely observation from Sikkema’s presentation was his revenue-and-cost comparisons. When he arrived at Ridgetown in 1993, the general thought was that if you had perfect weed control on your farm, you spent too much money on herbicide. “Thirty-three years later, I’ve completely changed that: your goal should be 100 per cent control and zero weed seed return to the soil,” he said. “The focus should be on new weed species that have a high propensity to evolve resistance and focus on those species that have the greatest impact on corn yield.” A weed control program should be planned field-by-field, and Sikkema started with Weed Spectrum #1 – green foxtail and barnyard grass, lamb’s quarters, redroot pigweed, common ragweed, wild mustard and wild buckwheat. Both Acruon and Primextra provide 95 per cent control of those weeds, yet Acuron costs $36.06 per acre (2025 suggested retail price) while Primextra costs $17.48, saving $13.58 per acre. With Weed Spectrum #2 (replacing wild mustard and wild buckwheat with velvetleaf and multiple-herbicide-resistant waterhemp), the use of Primextra’s low rate would cost a grower more. The herbicide only provides 25 per cent control of velvetleaf and has no activity on herbicide-resistant waterhemp. With this weed spectrum, Acuron, despite its $36.06 per acre price, provides 95 per cent control of the substituted weed species. When presented as a reflection of total farm income, the results are startling. Assuming a grower plants 1,000 acres of Enlist corn, one 500acre field has Weed Spectrum #1 and the other 500 has Weed Spectrum #2. Citing the Ontario Ministry of Agriculture, Food and Agribusiness (OMAFA) five-year average yield of 168 bu/ac and a price of $6.56 per bushel, gross revenue would be $1,102 per acre (see Table 1). But factoring the cost of Acuron versus Primextra and their effect on the two weed spectra, plus the cost of application ($12 per acre), the total weed management cost would be $29.48 per acre for Primextra and $48.06 for Acuron ($36.06 + $12). If a grower splits the use of the two based on field-specific weed spectra, it’s a difference of $61,000 in total farm income (Table 1). “If those weeds come up at the same time as the crop, they have a much greater impact on corn yield than if the weeds come up a week or two after the corn. Photo: Peter Sikkema continued on page 28
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