BIOLOGICAL YIELD AND HARVEST INDEX IN RICE - NITROGEN RESPONSE OF TALL AND SEMIDWARF CULTIVARS

The widespread adoption of high yielding, semidwarf, N-responsive rice (Oryza sativa L.) cultivars by California rice growers in the late 19 70s raised questions about improved fertilizer use efficiency and redu ced straw production of semidwarf rice cultivars under direct, water-s eeded cultural systems. The objective of this study was to determine w hether increased grain yields of semidwarf cultivars result from incre ased biological yield, improved partitioning (harvest index [HI]), imp roved N responsiveness (for example, improved fertility), or some comb ination of these factors. Twelve field experiments that included 12 cu ltivars were conducted from 1976 through 1985 at five locations, with preplant applied N rates ranging from 0 to 210 lb/acre. No differences in biological yield were observed between tall and semidwarf cultivar s across all N rates. Grain and straw production did differ significan tly, however, as a result of changes in HI across all N rates. The HI was 0.46 and 0.50 at predicted maximum grain yields, which occurred at 124 and 149 Ib N/acre for tall and semidwarf cultivars, respectively. Maximum predicted straw yields occurred at 216 Ib N/acre for semidwar f cultivars; but predicted maximum straw yields for tall cultivars occ urred far outside of the maximum N rates used in these experiments (at 245 Ib N/acre). At each N rate, semidwarf cultivars exhibited an impr oved HI over tall cultivars. Maximum yields for semidwarf cultivars oc cur at higher N rates than tall cultivars, however, which diminished i mproved HI values and offset potential reductions in straw yields unde r field conditions. Overall results indicate that future yield increas es are more likely to be the result of stabilization of HI over increa sing N rates, rather than an increase in biological yield.