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. [The study ran for four years concluding with the report in 2004].

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 four consecutive years 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 percent 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 seed bank at the conclusion of the four-year study was reduced by 25 percent to 63 percent with point-injected compared with broadcast N. Information gained in this study is contributing to the development of more integrated and cost-effective weed management programs in wheat.