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Basin wildrye [<i>Leymus cinereus</i> (Scribn. and Merr.) Á. Löve] and creeping wildrye [<i>Leymus triticoides</i> (Buckley) Pilg.] are native perennial grasses cultivated for seed used for fire rehabilitation and revegetation in western North America. Although <i>L. cinereus</i> produces large spike inflorescences with many seeds, it is prone to seed shattering. Seed can be harvested before shattering, but often displays poor germination and seedling vigor. Conversely, <i>L. triticoides</i> has fewer seeds per spike, but relatively strong seed retention. Both species are allotetraploid (2<i>n</i> = 4<i>x</i> = 28) and form fertile hybrids used for breeding and genetic research. A dominant, major-effect seed-shattering gene (<i>SH6</i>) from <i>L. cinereus</i> was previously identified in an <i>L. triticoides</i> backcross population. In this study, a DNA marker was used to select the recessive <i>L. triticoides</i> seed-retention allele (<i>sh6</i>) in cycle six (C6) of a <i>L. cinereus</i> × <i>L. triticoides</i> breeding population and evaluate gene × harvest date effects on seed yield and germination characteristics in a full-sib family derived from homozygous (<i>sh6</i>/<i>sh6</i>) and heterozygous (<i>SH6</i>/<i>sh6</i>) C6 parents. Although seed yields of shattering genotypes were 19.4% greater than non-shattering genotypes on the first harvest dates, yields of non-shattering genotypes were 167% greater on the last harvest dates. Seed harvested on the last harvest date reached 50% germination 4.2 days (26.4%) earlier and displayed 20.5% higher upper percentage germination limits than seed harvested on the first harvest date. Results indicate that the <i>sh6</i> seed-retention gene will improve basin wildrye seed retention and indirectly improve seed germination by enabling later harvest dates.