Irrigated wheat (Triticvm aestivum L.) in Texas is grown in early-planted production systems that are grazed by stocker cattle (Bos spp.) and late-planted nongrazed production systems. The objectives were to: (i) verify cultivar X production system interactions for 11 cultivars in 1988 and (ii) better understand grain yield relationships using data from 1986 experiments involving two semidwarf cultivars, Q588 and Vona, and the taller Siouxland. Average grain yield loss in 1988 due to grazing (grazed vs. nongrazed system) was 0.52, 1.37, and 2.18 Mg ha-1 for tall, intermediate, and short cultivars, respectively. Responses within height classes were consistent for three tall cultivars and five short cultivars. In the nongrazed production system in 1986, Q588 and Vona were better adapted as shown by linear increases in seed number and grain yield st anthesis biomass levels up to 1600 g m-2. In contrast, Siouxland a taller cultivar did not increase seed number or grain yield when biomass exceeded 1300 g m-2. Severe or later grazing reduces height, leaf area, and biomass. Under these less productive conditions, Siouxland yielded as much as Q588 or Vona. The cultivar X system interaction is caused by the greater response of semidwarf cultivars to productive nongrazed systems coupled with the size advantage of tall cultivars in grazed systems. As a result, the yield advantage of semidwarf cultivars over tall cultivars is much greater in nongrazed as compared to grazed systems.
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