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.

Information on nitrogen fertilizer effects on crop–weed competitive interactions might aid in developing improved weed management programs. A controlled environment study was conducted to examine the effect of three N rates on the competitive ability of four weed species grown with wheat. The four weed species were chosen to represent species that varied in their growth responsiveness to nitrogen (N): Persian darnel (low), Russian thistle (low), redroot pigweed (high), and wild oat (high). Wheat and each weed species were grown in a replacement series design at N rates of 60, 120, and 240 mg N kg−1 soil. The competitive ability of the low N-responsive species, Persian darnel and Russian thistle, was not influenced by N rate, supporting our hypothesis that N rate would have little effect on the competitiveness of species responding minimally to N. Conversely, the competitiveness of the high N-responsive species redroot pigweed progressively improved as N rate increased. However, wild oat competitiveness was unaffected by N fertilizer rate. There is some evidence from this study to suggest that fertilizer management strategies that favor crops over weeds deserve greater attention when weed infestations consist of species known to be highly responsive to higher soil N levels. Information gained in this study will be used to advise farmers of the importance of strategic fertilizer management in terms of both weed management and crop yield.

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.

Henbit is increasing in abundance in western Canada, and control recommendations are largely limited to herbicides. Increased knowledge of henbit biology may allow the development of more integrated control programs. A controlled environment study was conducted to determine the combined effect of various soil temperature and soil water levels on the emergence of henbit. Henbit emerged at soil temperatures ranging from 5 to 25 C, but the highest emergence of 81 to 83% occurred at 15 to 20 C. Henbit emergence declined as soil water content decreased. The interaction of cool and dry soils caused the greatest inhibition of henbit emergence. At progressively lower soil water levels of −0.03, −0.28, −0.53, −0.78, −1.03, and −1.53 MPa, henbit emergence was 78, 61, 64, 40, 38, and 11% at 10 C, respectively. Rate of henbit emergence was affected less by soil water than by soil temperature. A decrease in soil water content from −0.03 to −1.53 MPa increased the time to reach 50% emergence (ET50) by 1 to 5 d, whereas a decrease in temperature from 25 to 5 C increased the time to reach ET50 by 13 to 16 d. The implications of these results in terms of improved management of henbit are discussed.

In summer, 2011, we investigated suspected glyphosate-resistant (GR) kochiain three chem-fallow fields (designated F1, F2, F3, each farmed by adifferent grower) in southern Alberta. This study characterizes glyphosateresistance in those populations, based on data from dose–responseexperiments. In a greenhouse experiment, the three populations exhibited aresistance factor ranging from 4 to 6 based on shoot biomass response (GR50 ratios), or 5 to 7 based on survival response (LD50 ratios). Similar results were found in a fielddose–response experiment at Lethbridge, AB, in spring 2012 using the F2kochia population. In fall 2011, we surveyed 46 fields within a 20-km radiusof the three chem-fallow fields for GR kochia. In the greenhouse,populations were screened with glyphosate at 900 g ae ha−1. Sevenpopulations were confirmed as GR, the farthest site located about 13 km fromthe three originally confirmed populations. An additional GR population morethan 100 km away was later confirmed. Populations were screened foracetolactate synthase (ALS)–inhibitor (thifensulfuron : tribenuron) anddicamba resistance in the greenhouse, with molecular characterization ofALS-inhibitor resistance in the F1, F2, and F3 populations. All GRpopulations were resistant to the ALS-inhibiting herbicide, but susceptibleto dicamba. ALS-inhibitor resistance in kochia was conferred by Pro197, Asp376, or Trp574 amino acidsubstitutions. Based upon a simple empirical model with a parameter forselection pressure, calculated from weed relative abundance and glyphosateefficacy, and a parameter for seedbank longevity, kochia, wild oat, andgreen foxtail were the top three weeds, respectively, predicted at risk ofselection for glyphosate resistance in the semiarid Grassland region of theCanadian prairies; wild oat, green foxtail, and cleavers species werepredicted at greatest risk in the subhumid Parkland region. This studyconfirms the first occurrence of a GR weed in western Canada. Futureresearch on GR kochia will include monitoring, biology and ecology, fitness,mechanism of resistance, and best management practices.