Optimal Wheat Seeding Rates for Conventional and Narrow Rows for Cheat-Free and Cheat-Infested Fields

Serious cheat (Bromus secalinus L.) infestations often result after several years of conservation tillage in continuously cropped winter wheat (Triticum aestivum L.) fields in the U.S. Southern Great Plains. This study was conducted to determine profit maximizing seeding rates for conventional and narrow row production systems for both cheat-free and cheat-infested fields. Maximum likelihood methods were used to estimate wheat response functions. The profit maximizing seeding rate was relatively insensitive to the price of seed and the price of wheat. However, optimal seeding rates were found to be greater in cheat-infested plots. Reducing row spacing from conventional (9 in.) to narrow (3 in.) rows, with the seeding rate held constant, resulted in increased yields in both cheat-free and cheat-infested plots. It is appropriate to recommend that seeding rate be increased in fields that are heavily infested with cheat. However, the cheat problem can not be solved by changing either row spacing or seeding rate. Research QuestionWeedy grass species that are physiologically similar to wheat, such as cheat, often become serious problems in continuously cropped winter wheat fields. In general, a heavy cheat infestation will result if stubble mulch tillage is used for several years in fields cropped continuously to wheat in the Southern Great Plains. On the other hand, very little cheat would be expected in a field that is plowed with a moldboard every year. The inability to control cheat has been a primary impediment to the adoption of continuous conservation tillage systems. The objective of this research was to determine the impact of cheat infestation, row spacing, and seeding rate on wheat yield and to determine profit maximizing wheat seeding rates for cheat-free and cheat-infested fields. Literature SummaryStudies have been conducted to evaluate the relationship between row spacing and wheat grain yield. The most efficient spacing of wheat seeds is obtained when seeds are placed in a rhomboidal pattern such that the distance between seeds within the row is equal to the distance between rows. For seeding rates and varieties common in the Southern Plains, row spacing as narrow as 2.0 to 2.5 in. would be necessary to achieve a rhomboidal pattern. Studies have also determined that wheat yield is a function of seeding rate. However, the influence of seeding rate and row spacing on wheat yield for alternative levels of cheat infestation has not been established. It is not known if the optimal seeding rate is different in cheat-infested fields. Study DescriptionTwenty field experiments, with row spacing, seeding rate, and cheat level as treatment variables, were conducted between 1988 and 1991 at 14 different locations in Oklahoma. A total of 460 treatments were planted and harvested. Wheat seeding rates varied from 60 to 120 lb/acre. Row spacing ranged from 9 to 3 in. Cheat infestation levels were artificially induced by seeding from zero to 120 lb/acre of cheat seed. Maximum likelihood was used to estimate the impact of cheat level, row spacing, and seeding rate on wheat grain yield. Applied QuestionsDoes cheat reduce wheat yield? The cheat infestation that resulted from seeding 90 lb/acre of cheat seed reduced predicted wheat yield by 3 to 7 bu/acre depending upon wheat seeding rate and row spacing. For 9 in. rows and a 90 lb/acre seeding rate, a cheat infestation of 120 lb/acre reduces the predicted wheat yield from 30 to 18 bu/acre (40%). These results confirm the very serious consequences of the weed and explain why farmers are concerned about cheat. Will narrow rows result in greater yields? Reducing row spacing from 9 to 3 in. is expected to increase wheat yield by 7 to 10%. What is the optimal seeding rate? For a given level of cheat infestation and row spacing, the profit maximizing seeding rate is a function of the price of seed and the price of wheat. The range of estimated optimal seeding rates in the region is from 72 lb/acre for high wheat seed prices ($0.15/lb), low wheat grain prices ($2.50/bu), and no cheat, to 133 lb/acre for low seed prices ($0.075/lb), high wheat grain prices ($4.00/bu), and heavy cheat infestation. Is the optimal seeding rate different in cheat-infested fields? Profit maximizing seeding rates are sensitive to the level of cheat infestation. Wheat seeding -rates should be increased in cheat-infested fields. The profit maximizing wheat seeding rate for fields that have cheat infestation similar to that resulting from seeding 120 lb/acre of cheat is approximately 41 lb/acre more than that for cheat-free fields. Even with the increased seeding rate, however, yield losses exceeding 10 bu/acre (30%) can be expected from severe infestations of cheat. Can the cheat problem be solved by adjusting seeding rate and row spacing? Reducing row spacing from conventional (9 in.) to narrow (3 in.) rows, with the seeding rate held constant, resulted in increased yields in both cheat-free and cheat-infested fields. It is appropriate to recommend that seeding rate be increased in fields that are heavily infested with cheat. However, the cheat problem cannot be solved by either reducing row spacing or increasing seeding rate.