The response of susceptible (S) and resistant (R) green foxtail biotypes to increasing dosages of trifluralin, applied PPI in rapeseed and preemergence incorporated (PEI) in wheat, was investigated in field experiments in 1989 and 1990. Differences in response between the biotypes to PPI- and PEI-trifluralin were 7- and 12-fold, respectively, based on density and shoot biomass determinations 4 wk after emergence. Nine- and 14-times higher dosages of PPI- and PEI-trifluralin, respectively, were required to reduce R-seed production by 50% than to reduce S-seed production by the same amount. At the recommended trifluralin dosage in rapeseed (1.4 kg ha−1), the density of S-plants 4 wk after emergence was reduced by 84% compared with untreated plots, whereas the density of R-plants was reduced by only 4%. The effective kill (seed yield reduction) was 99% and 42%, respectively. At the recommended dosage in wheat (0.9 kg ha−1), the density of S-plants 4 wk after emergence was reduced by over 99% compared with less than 36% for R-plants. The effective kill was 97% and 14%, respectively. Based on determination of effective kill, the selection pressure of trifluralin on green foxtail is greater when the chemical is applied PPI in rapeseed than when applied PEI in wheat, even though initial density reductions are less in the former than the latter.

The comparison of incorporation vs no incorporation on a mid-October application of the granular formulation of trifluralin to control infestations of wild oat and green foxtail was evaluated in spring barley, wheat, and canola in west central Saskatchewan over a 3 yr period. Incorporation treatments consisted of: no incorporation, one incorporation in fall, one incorporation in fall and a second in spring, one incorporation in fall and two additional incorporations in spring. All treatments eliminated green foxtail. In all three crops wild oat panicle counts were equivalent from incorporated and non-incorporated trifluralin. Wild oat fresh weights in crops grown on stubble were similar for incorporated and non-incorporated trifluralin. In fallow crops, wild oat fresh weight reductions were greater in three of nine site years with incorporation. There was little difference in crop yields from incorporated and non-incorporated trifluralin.

Dose-response experiments showed that three samples of green foxtail collected from fields in southern Manitoba where trifluralin had not controlled the weed in 1987 were resistant to the herbicide. GR50 values indicated that the resistant populations were about 5 times more resistant to trifluralin than susceptible populations, about twice as resistant as wild oat, and about equally resistant as wheat. An additional 33 samples collected in 1988 from southwestern Manitoba where trifluralin has been used for nearly 20 yr in both cereal and oilseed crops were resistant to trifluralin.

Round-leaved mallow interference in spring wheat and flax was investigated under field conditions in southern Manitoba. Additionally, the growth of mallow alone and in a wheat crop was compared. In six of seven trials, wheat yield losses due to mallow interference were not significant. In a seventh trial, where mallow densities of up to 237 plants m−2 were recorded, wheat yields were reduced up to 15%. In contrast, significant flax yield losses occurred in each of three trials where maximum mallow densities ranged from 21 to 52 plants m−2. From the regression equations it was calculated that 20 mallow plants m−2 caused flax yield losses of 33, 9, and 10% in 1987, 1988, and 1989, respectively. Periodic sampling of mallow plants growing alone and in wheat, and regression analysis of these data using a logistic function as a biological model of plant growth indicated that mallow growth was severely suppressed by wheat By the eighth week after emergence, the lamina area and shoot dry matter of mallow plants growing in wheat was less than 3% of plants growing alone. Similarly, in wheat, mallow produced less than 1% of the seed produced by plants growing alone. This extreme suppression of mallow growth corresponded with an 80 to 90% decrease in photosynthetically active radiation penetrating the wheat canopy for approximately six continuous weeks, beginning 4 wk after crop emergence.

Information on weed responses to soil fertility levels is needed to aid development of fertilizer management strategies as components of integrated weed management programs. A controlled environment study was conducted to determine shoot and root growth response of 22 agricultural weeds to fertilizer phosphorus (P) applied at 5, 10, 20, 40, or 60 mg kg−1 soil. An unfertilized control was included. Wheat and canola were included as control species. Shoot and root growth of all weeds increased with added P, but the magnitude of the response varied greatly among species. Many weeds exhibited similar or greater responses in shoot and root biomass to increasing amounts of soil P compared with wheat or canola. With increasing amounts of P, 17 weed species increased shoot biomass more than wheat, and 19 weed species increased shoot biomass more than canola. However, only 10 weed species exhibited greater increases in root biomass than canola, and no weed species increased root biomass more than wheat with added P. Canola was among species taking up the greatest percentage of available P at all P doses. However, percentage P uptake by wheat relative to other species varied with P dose. Only four weed species extracted more P than wheat at low P levels, but 17 weed species extracted more P at high soil P levels. These findings have significant implications as to how soil fertility may influence crop–weed competition.

Improved fallow systems are needed to reduce the negative effects of increased soil erosion and reduced soil quality. Field experiments were conducted to determine weed suppression attained with yellow sweetclover grown as a green manure fallow replacement crop. Yellow sweetclover was undersown in field pea, flax, or Indian mustard and then killed in June of the following fallow year. Living yellow sweetclover competed strongly with weeds during the first fall and spring of fallow. Weed biomass accounted for <1 to 12% of the total plant biomass when yellow sweetclover was terminated in June. Yellow sweetclover residues remaining after termination of growth continued to provide excellent weed suppression. Weed densities in April before planting the succeeding wheat crop were 75 to 97% lower in yellow sweetclover than in untreated fallow treatments. Yellow sweetclover controlled the perennial weeds dandelion and perennial sowthistle, as well as the annuals kochia, flixweed, Russian thistle, and downy brome. Weed suppression was similar whether yellow sweetclover was harvested as hay or its residues were incorporated or left on the soil surface, suggesting that a portion of the weed suppression effect may be due to allelopathic compounds being released from decomposing yellow sweetclover. Results will be used to develop more sustainable agronomic practices in regions where fallow is still widely employed.

Information on responses of weeds to various soil fertility levels is required to develop fertilizer management strategies as components of integrated weed management programs. A controlled environment study was conducted to determine shoot and root growth response of 23 agricultural weeds to N fertilizer applied at 0, 40, 80, 120, 180, or 240 mg kg−1 soil. Wheat and canola were included as control species. Shoot and root growth of all weeds increased with added N, but the magnitude of the response varied greatly among weed species. Many weeds exhibited similar or greater responses in shoot and root biomass to increasing amounts of soil N, compared with wheat or canola. With increasing amounts of N, 15 weed species showed a greater increase in shoot biomass, and 8 species showed a greater increase in root biomass, compared with wheat. Ten weed species exhibited increases in shoot biomass similar to that exhibited by canola, and five weed species showed greater increases in root biomass than did canola, as N dose was increased. All crop and weed species extracted > 80% of available N at low soil N levels. At the highest N dose, 17 of 23 weed species took up similar or greater amounts of soil N than did wheat, and 6 weed species took up N in amounts similar to that taken up by canola. These findings have significant implications as to how soil fertility affects crop–weed competition. The high responsiveness of many weed species to N may be a weakness to be exploited through development of fertilizer management methods that enhance crop competitiveness with weeds.

Managing fertilizer in cropping systems may be an important component of integrated weed management programs. A field study was conducted to determine the effect of various application methods of the 15N-enriched nitrogen (N) fertilizer on N uptake in green foxtail, wild mustard, and spring wheat and on crop–weed competition. N application methods consisted of ammonium nitrate in solution applied broadcast on the soil surface, applied in pools on the soil surface at 20-cm intervals between every second wheat row, and point injected 10 cm deep at intervals similar to those of the surface pools. An unfertilized control treatment was also included. N uptake by green foxtail throughout the growing season was often greater from surface broadcast than from surface pools or point-injected N and was sometimes greater from surface pools than from point-injected N. In contrast, N uptake by wild mustard was rarely affected by the fertilizer placement method. In the presence of weeds, the ranking of N uptake by wheat usually was point injected > surface pools > surface broadcast. Weed biomass was often greater with surface broadcast than with either surface pools or point-injected N. In the absence of weeds, wheat yields were similar across the three N application methods. However, in the presence of green foxtail, wheat yields were greater with point-injected N than with surface broadcast N in two of the 3 yr and with surface pools of N in one of the 3 yr. In the presence of wild mustard, wheat yields were greater with surface pools and point-injected N compared with the unfertilized control in two of the 3 yr, whereas yields with broadcast N were never greater than the unfertilized control. Study findings suggest that point-injected N results in suppressed weed growth, not by reduced N uptake by weeds but instead by greater N uptake by wheat that increases its competitiveness with weeds. Information gained in this study will be utilized to develop a more integrated program for weed management in spring wheat.

Managing crop fertilization may be an important component of integrated weed management systems that protect crop yield and reduce weed populations over time. A field study was conducted to determine the effects of various timings and application methods of nitrogen (N) fertilizer on weed growth and spring wheat yield. Nitrogen fertilizer was applied the previous fall (October) or at planting (May) at a dose of 50 kg ha−1. Nitrogen application treatments consisted of granular ammonium nitrate applied broadcast on the soil surface, banded 10 cm deep between every crop row, banded 10 cm deep between every second crop row, or point-injected liquid ammonium nitrate placed between every second crop row at 20-cm intervals and 10 cm deep. Treatments were applied in 4 consecutive yr to determine annual and cumulative effects over years. Density and biomass of wild oat, green foxtail, wild mustard, and common lambsquarters were sometimes lower with spring- than with fall-applied N. Spring wheat yield was never lower and was higher in 50% of the cases, when N was spring rather than fall applied. Nitrogen application method generally had larger and more consistent effects than application timing on weed growth and wheat yield. Shoot N concentration and biomass of weeds were often lower with subsurface banded or point-injected N than with surface broadcast N, and concurrent increases in spring wheat yield usually occurred with these N placement treatments. Depending on the weed species, the weed seedbank at the conclusion of the 4-yr study was reduced by 25 to 63% with point-injected compared with broadcast N. Information gained in this study will contribute to the development of more integrated and cost-effective weed management programs in wheat.