Annual weeds, their viable seed population in the soil, and their effect on yields of oats, wheat, and flax

Two methods of determining soil weed seed population were studied in 1944. a. Four small samples, each consisting of 19 plugs of soil, were obtained from each of 24 plots, placed in greenhouse flats, and the seeds present allowed to germinate. b. Two large samples, each consisting of 154 plugs of soil, were obtained from each of 24 plots, washed through sieves, and the remaining material placed on blotters in seed germinators. The small sample method gave higher weed counts than the large sample method. At every location there was less variability between the large samples than between the small samples taken from the same plot. The effect of annual weeds on yields of small grain and flax was studied from 1944 to 1946, 3 years in which moisture was not a major limiting factor to grain yield at St. Paul. In 1944 a moderate infestation of annual weeds reduced the yields of oats, wheat, and flax significantly. Complete weed removal when the crop was about 4 inches tall frequently gave an increase in yield over weedy crops nearly equal to that obtained from weed-free crops. In 1945 no significant effect of weeds on crop yield was obtained. Plant tissue tests showed nitrate deficient in plants from both weedy and weed-free plots. In 1946 the comparative yields of flax infested with Setaria species, infested with lamb's quarters-wild buckwheat-grass, and under weed-free conditions were studied at nine dates of harvest. Yields of flax plants and of flax straw were highest under weed-free conditions and lowest with the lamb's quarters-buckwheatgrass infestation. On August 9 the flax with the lamb's-quarters-buckwheat-grass infestation produced the lowest yields of seed, but the differences in yields of flaxseed were not significant. Yields of Setaria plants were negatively correlated with yields of flax plants at each of nine harvests from June 11 to August 9 and with yields of flaxseed and of flax straw on August 9. A short weed cover between the drill rows of flax resulted in a cooler soil and more soil moisture than with flax weed-free.