US Fish & Wildlife Service


Date of this Version



Proceedings, Western Section, American Society of Animal Science (60) 2009


The recruitment of calves at the National Bison Range (NBR) near Moiese, MT has dropped from the historic average of 87 to 33 calves per 100 breeding age cows in 2008. The purpose of this study is to monitor the NBR bison pregnancy rates and calf recruitment in an effort to determine where in the reproductive cycle NBR female bison fail to recruit calves. The reproductive cycle was divided into 3 stages: conception to early embryonic development; maintenance of pregnancy during the second and third trimesters; and, calving to recruitment. Herein we report results related to the first two stages. Pregnancy rate of 41 cows, ages 4 to 12 yr, were determined by transrectal ultrasonography of the uterine contents of each cow in early October at the NBR’s annual roundup. Pregnant cows were given a number, painted on the right flank of each animal, using a commercial hair-bleaching agent. Numbers were used to identify individual cows throughout the study period. A blood sample, collected by jugular venepuncture and fecal sample were collected from each cow and from 3 bulls for assay of progesterone (P4) by RIA. Serum and fecal P4 concentrations from anovular cows and bulls were used to determine a P4 baseline to evaluate pregnancy rate using P4 concentrations in fecal samples collected from cows, collected in the field, in early January. The criterion used for evaluating pregnancy was based on fecal P4 concentrations of bulls and anovular cows plus 2 SDs or 20.4 ng/g of feces. Additional fecal samples were collected from cows that were not painted with a number to better estimate the overall pregnancy rate of the herd. During the 2008 roundup, 28 of 41 (68%) cows were determined to be pregnant. During the second trimester samples were collected from 26 of the 28 numbered cows and 28 unmarked cows. Of those, 15 (58%) numbered and 4 (14%) unmarked cows had fecal progesterone concentrations that exceeded the criterion for a P4 concentration considered to be consistent with pregnancy. By establishing the timing of reproductive failures, managers will be able to focus their efforts in determining the causative agent(s) of the reproductive failure in this herd.