USING UPPER-BOUND SLOPE THROUGH ORIGIN TO ESTIMATE GENETIC HARVEST INDEX

Harvest index (HI), the ratio of grain to aboveground dry matter, is reported to be a species-related parameter and is recommended for screening cultivars. But the fact that it is affected by environmental stress limits its use for intercrop or intercultivar comparisons. Fair comparisons should be based on estimated genetic HI for a given environment, but a procedure to determine the same is lacking. We propose that the slope of an upper-bound in the grain yield vs. dry matter plot passing through the origin approximates the genetic HI because the highest grain yields against given dry matter represent the least-stressed and/or stress-adapted plants and passage of the line through the origin is necessary to satisfy the definition of HI. This HI also provides a useful reference for interpreting agronomic data with respect to stress effects associated with management practices. The HIs of sorghum [Sorghum bicolor (L.) Moench], corn (Zea mays L.), and wheat (Triticum aestivum L.) were estimated by the upper-bounds of grain yield (corrected to dry weight) vs. dry matter yield taken from existing reports in the literature. Harvest index of sorghum and irrigated corn ranged between narrow limits of 0.48 to 0.53 and 0.58 to 0.60, respectively. Harvest index of irrigated wheat ranged from 0.38 to 0.47. Stress effects on HI are illustrated in plots of published and unpublished (dry) grain yields vs. dry matter yields and reasons for the same are discussed.