Field experiments were conducted in 1996, 1997, and 1998 at Lacombe and Lethbridge, AB, to determine the influence of early weed competition on field pea yields. Wild oat and Tartary buckwheat were removed from plots at weekly intervals after pea emergence by hand-weeding and maintained weed-free for the remainder of the growing season. Tartary buckwheat produced slightly more biomass than wild oat at Lacombe, whereas wild oat produced much more biomass than Tartary buckwheat at Lethbridge. Weed-free pea yields at Lacombe were always two- to threefold higher than at Lethbridge. At Lethbridge, early competition with Tartary buckwheat in all years, and with wild oat in 1998, did not reduce pea yields. In 1996 and 1997 at Lethbridge, pea yield reductions due to wild oat began at 2 wk after pea emergence; for the next 2 wk yield loss was linear, decreasing at an average rate of 97 kg/ha per day. Early weed competition led to pea yield losses at Lacombe in all 3 yr. The onset of yield loss at Lacombe over the 3-yr period ranged from 1 to 2 wk after pea emergence; for the next 2 to 3 wk yield loss was linear, decreasing at an average rate of 45 kg/ha per day. Yield losses after full-season weed competition ranged from 40 to 70% at both sites. Usually, the beginning of the critical weed-free period was at 1 or 2 wk after pea emergence. Optimum pea yields usually required weed removal very early in the pea life cycle; weed removal beyond 2 wk after pea emergence often protected only suboptimal yields.

Knowledge of the soil nitrogen (N) supply and the N mineralization potential of the soil combined with an understanding of weed-crop competition in response to soil nutrient levels may be used to optimize N fertilizer rates to increase the competitive advantage of crop species. A greenhouse study (2006) and field studies (2007 to 2008) in Illinois and Nebraska were conducted to quantify the growth and interference of maize and velvetleaf in response to varying synthetic N fertilizer rates in soils with high and low N mineralization potential. Natural soils were classified as having “low mineralization potential” (LMP), while soils amended with composted manure were classified as having “high mineralization potential” (HMP). Maize and velvetleaf were grown in monoculture or in mixture in both LMP and HMP soils and fertilized with zero, medium, or full locally recommended N rate. In the greenhouse, velvetleaf interference in maize with respect to plant biomass increased as N rate increased in the HMP soil, whereas increasing N rate in the LMP soil reduced velvetleaf interference. In contrast, velvetleaf interference in maize decreased as N rate increased regardless of soil class in the field experiment. With respect to grain yield, velvetleaf interference in maize was unaffected by N rate or soil class. In both greenhouse and field experiments, velvetleaf biomass was greater in the HMP soil class, whereas maize interference in velvetleaf was generally greater in the LMP soil class. While soil N levels influenced weed-crop interference in the greenhouse, the results of the field study demonstrate the difficulty of controlling soil nutrient dynamics in the field and support a maize fertilization strategy independent of weed N use considerations.