There are only 365 days (except leap year)
in any year. If a beef cow is to have a calf every year and pay
her way in the herd, she has to return to heat, conceive, gestate and
deliver a baby on a 12 month schedule. We often think of gestation
length as a constant. Most gestation tables list it as 283 days.
However, variation in gestation lengths do occur. In fact, data
from the USDA-Meat Animal Research Center lists variation in gestation
length between sire breeds of nearly 12 days. Consequently the longer
gestation cattle have fewer days between calving and the next conception
if they are to stay on a twelve month calving inteval. The graphic
below from Denham, and co-workers in 1991 illustrates the effects of pro-longed
gestation lengths on calf crop percentage in a commercial beef herd. 
Cows that gestate in excess of 290 days must return to heat very soon
(at or before 60 days after calving) to maintain the same reproductive
performance of shorter gestation cows. This means that these "long
gestation cows" must be in very good body condition (BCS =6 or 7) at calving
time to have an opportunity to rebreed on time. Gestation lengths
do vary between breeds and within breeds. Commercial producers need
to understand the impacts that long gestation lengths can have on their
herd's productivity. In addition, commercial producers should continue
to ask for more information from breeders and breed associations about
gestation lengths reported for the seedstock cattle that they purchase.
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