The onslaught of a wet, cold winter, several blizzards, and unbelievable flooding has caused some Midwest cattle producers to re-examine the timing of future calving seasons. There will be popular press and social media articles suggesting that calving seasons need to be moved to late spring and early summer. For those regions of the country prone to late winter, spring snowstorms and blizzards, moving the calving season out of these stressful weather events makes sense.
If the calving season is moved to May and June, then the breeding season must be moved to August and September. In the upper Midwest, breeding seasons in the hotter months of summer may be feasible. Although, 90 to 100 degree days may occur, nighttime temperatures will often cool to 70 degrees or lower. However in Oklahoma and Texas, in August daytime temperatures often reach near or above triple digits and night time lows may only cool about 80 degrees. A high pressure heat dome may lock in very hot days and warm nights for an extended period of time. The number of hours each day that the temperature is above the thermal neutral maximum (80 degrees in the bovine) is sizeable. There is little if any opportunity for the cow to dissipate heat in this scenario. Therefore heat stress becomes a biological nemesis to good reproductive performance in late summer months in Oklahoma.
Research conducted several years ago at Oklahoma State University (Biggers, et al, 1986 OSU Animal Science Research Report) illustrated the possible impact of heat stress of beef cows on their reproductive capability. They found that heat stress of beef cows from day 8 through 16 after breeding affected the weights of the conceptus (embryo, fluids, and membranes) and the increased body temperature may have formed an unfavorable environment for embryo survival. The percentage of pregnancies maintained throughout the week of severe heat stress was considerably reduced (83% for non-heat stressed versus 50% for severely heat stressed).
Also research (Meyerhoffer, et al 1985. J. Animal Science 60:352) has clearly shown that semen quality of heat stressed bulls will be significantly reduced and will take nearly 8 weeks after the heat stress to fully recover. When reduced fertility in male is multiplied by reduced embryo survival in the female, percentage calf crops must decline.
Heat stress causes a percentage decrease in pregnancy percentages. It is not an “all or nothing” situation. Spring breeding seasons starting in May and finishing in late June should avoid most of the heat stress in Oklahoma. This results in February and March calving seasons. Fall calving (with breeding seasons beginning in late November and ending in January) allow for fertilization and early embryonic survival when heat stress is not a factor. These calves arrive starting in early September.
The best choice for calving seasons will be different for the different climates and weather patterns in the United States.