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Degree-day accumulation is commonly used to predict crop development and harvest dates. Relationships between degree-day accumulation and phenological development of range forage grasses have received less attention. This research tested the hypotheses that leaf development by big bluestem [Andropogon gerur-&Vitman] and little bluestem [Scfiac~yrium scopuriuu~ (Michx.) Nash] is related to degree-day accumulation and that these relationships are stable over environments and years within environments. Study environments included native prairie, a spaceplanted garden, and a growth chamber. Individual tillers of big and little bluestem were permanently marked and fully developed leaves were counted once or twice weekly over 3 growing seasons and 1 growth chamber trial. Quadratic regression models accounted for 94 to 99% of the observed variation in leaf development for all species-environment-year combinations. Regression models were significantly different (p= 0.05) among environments and between years within environments. Lack of model stability over years was a result of high variation in total leaves produced per tiller relative to annual variation in degree-day accumulation. The simple lndo pendent variable, day of year, predicted leaf development equally as well as degree-day accumulation.

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Herbage production in the California annual grassland has been correlated with seasonal weather, particularly fall and spring rainfall patterns. To further examine the relationship between herbage production and rainfall pattern, 3 soil water regimes (-1,-7,-15 bars) simulating expected rainfall and drought events in annual rangelands were applied in seminatural annual grassland communities. Soft chess (Bromus mollis) tillers grew longest under the-7 bar water regime treatments while total plant growth was greatest under the-1 bar treatment. Tiller length and total growth of slender oat (Avenu burbata) were greatest under the-1 bar tredment. Vegetative growth of slender oat was less sensitive to seasonlong soil water regimes than soft chess. The two species required different soil water conditions for maximum spring growth; soft chess put on spring growth most rapidly in the-7 bar treatment while slender oat grew fastest in the-1 bar treatment. Periodic water stress during the growing season did not reduce spring herbage production. Maximum growth and herbage production occurred only when soil water was available after March 15. Withholding water after March 15 reduced herbage production by 46%.