Effects of phosphorus on Italian ryegrass (Lolium multiflorum Lam.) growth and alternative herbicide management for diclofop-resistant Italian ryegrass in winter wheat (Triticum aestivum L.)

Included within overall goals that attempt to determine the relationship between soil fertility, especially phosphorus, and relative growth of Italian ryegrass and winter wheat and elucidate control programs for diclofop-resistant Italian ryegrass, a field study was conducted to evaluate the effect of residual levels of soil phosphorus on a natural population of Italian ryegrass plants growing in winter wheat fields. The sustained increases in plant height, tiller number, and number of individuals m-2 as soil P levels increased to 40 mg dm-3, and in seedheat/m2 with increases of P levels to 55 Mg dm--3 suggested a positive trend of Italian ryegrass growth as soil residual P levels increased. Above these P levels those dependent variables fluctuated greatly, which may be attributed to effects of wheat-Italian ryegrass interactions. The most significant effect was illustrated with plant height. Greenhouse experiments with plants grown individually and in pairwise mixture compared differential growth characteristics and phosphorus utilization efficiency of Italian ryegrass and winter wheat over a range of phosphorus solution concentration (0.05, 0.25, 0.5, 0.75, and 1.0 m.M P). Height, number of tillers, leaf area, and dry matter production increased in both species in response to nutrient increase for both growing conditions. Both species showed optimal growth at 0.5 m MP solution concentration when grown individually. When grown in pairwise mixture, Italian ryegrass reached optimal growth in a range of 0.25--0.5 mMP whereas wheat required 0.5--0.75 mMP. The P level for optimal growth was concomitant with P solution concentration at which species reached their higher phosphorus utilization efficiency. Italian ryegrass exhibited less dry weight accumulation and phosphorus utilization efficiency than wheat. Yet, Italian ryegrass had higher relative growth rate than wheat regardless of P treatments, started to reach similar height and leaf area as wheat, and overcame wheat on number of tillers as P levels increased. Overall, species had similar response to P levels. During the period of study, Italian ryegrass competitiveness relative to wheat was not enhanced with increased P supply. However its relative growth rate, tillering, ability, and increasing plant height in response to P levels could be associated with onset of Italian ryegrass competitiveness in later growth stages. Evaluation of diclofop-resistant Italian ryegrass response to different rates, application timing, and formulation of trifluralin indicated that emulsifiable concentrate of trifluralin applied postplant incorporated at 280, 560, and 840 g ai ha-1, and trifluralin applied preplant incorporated at 560 and 840 g ai ha-1 provided excellent Italian ryegrass control during the 1996--1997 winter wheat growing season. There was no crop stand reduction. Wheat yields averaged 3762 kg ha-1. In the 1997--1998 growing season, emulsifiable concentrate of trifluralin applied postplant incorporated at 840 and 1120 g ha-1 provided fair Italian ryegrass control and a wheat stand reduction of 33%. Emulsifiable concentrate of trifluralin applied preplant incorporated, and trifluralin granular applied preemergence were ineflective.