Field experiments were initiated in 1973 at Fargo and Williston, ND, to evaluate wild oats seed longevity in soil at six burial depths. The number of viable seed decreased rapidly during the first 7 mo from 99% to 21 and 15% at Fargo and Williston, respectively, when averaged over depths. The number of viable seed decreased gradually over the 7- to 168-mo period at Fargo; however, at Williston, viability remained relatively constant for 84 mo and decreased rapidly after 108 mo. At both locations, several wild oats seed were viable 168 mo after burial. More viable wild oats seed were recovered from deep than shallow depths regardless of sampling date. Seed viability at the 6- to 10-cm burial depth was lost more rapidly with nitrogen fertilizer than without nitrogen fertilizer. The percentage of dormant viable seed remained constant for 33 mo at Fargo and 60 mo at Williston regardless of burial depth, but decreased gradually thereafter. Dormant seed percentages at the various depths did not relate to differences in seed viability at these depths.

Competitive interactions of wheat and wild oats in relation to variations in plant density of both species were investigated in field studies under irrigation and added fertilizer. Competition studies included comparisons by both additive and replacement series. The additive series was used to study plant responses to competition under fixed density of wheat and increasing density of wild oats. The replacement series was used to study plant responses to competition under constant total plant density with differing proportions of wheat and wild oats. On a per plant basis, shoot dry weight and leaf area index of wild oats were less than those of wheat at anthesis. However, the replacement series experiment indicated that wheat and wild oats were equivalent in competitiveness. Relative density of wild oats gave a better regression fit for wheat yield than did absolute wild oats density (r2 values of 0.83 and 0.61, respectively). Yield of wheat grain was linearly proportional to relative density of wild oats.

Prediction of the developmental stages of wheat and wild oats would be useful in order to: 1) correctly time the application of herbicides, and 2) accurately schedule research and cultural operations. The Haun developmental scale which numbers leaf development and describes floral development on the main stem of grasses was found to be suitable for describing the development of semidwarf wheat and wild oats in California. Haun developmental rates of wheat and wild oats were similar. Interference by wheat or wild oats in mixed cultures did not change the developmental rate of either species when grown with added nutrients and water. Degree days gave better correlations with development than calendar days when different planting dates, years, and locations were compared. A degree day model with a 5 C base temperature and a second-order polynomial expression gave accurate predictions of developmental stage, which correlated well with field data.