Oklahoma State University researchers used five years of data from the North Lake Carl Blackwell range to answer the question of gestation length differences by seasons. Records of 414 gestations and live births (242 spring and 172 fall) from cows of five crossbred cow groups were analyzed for differences in gestation length and birth weight.  The cows ranged in age from 4 to 7 years old.  Cows were bred artificially to either Salers or Limousin bulls.  The last breeding date was used to calculated gestation length.  Birth weights were taken within 24 hours of birth.  Fall calving cows delivered smaller birth weight calves (77.7 pounds) than did spring calving cows (82.2 pounds).  However gestation length was not affected by season.  Both fall and spring calving cows had an average gestation length of 284.4 days.  Source: Selk and Buchanan, 1990 OSU Animal Science Research Report