The reproductive performance of bison at the National Bison Range

Abstract

Bison (Bison bison) calf production at the National Bison Range (NBR) near Moiese, MT has dropped from the historic average of 87 to 32 calves per 100 breeding-age cows in 2008. The purpose of this project was to determine specific stages of the reproductive cycle of female NBR bison that may be responsible for low reproductive rate and calf production. The reproductive cycle was divided into 3 stages: conception to early embryogenesis; gestation; and perinatal calf survival. From 2008 to 2010, we used transrectal ultrasonography to determine pregnancy rate in cows ages 3 to 12 yr in October of each year. Sixty-eight percent (28/41), 63% (56/89) and 71% (50/70) were pregnant in 2008, 2009 and 2010, respectively. In 2008, pregnant cows were painted with a unique number using hair bleach. Fecal samples collected in Oct. 2008, and Jan. and Mar., 2009, were analyzed for progesterone (P4) concentrations. Because bleach marks did not persist through calving, in 2009 radio collars were secured to 27 pregnant and 10 non-pregnant cows. Fecal samples collected in Oct. 2009, and Jan., March, Apr., and May, 2010, were analyzed for P4 concentrations. Based on fecal P4 analyses, 26% of cows pregnant in October 2008 lost their pregnancy during the winter 2009. In 2009, 26 pregnant cows retained their collars. Of these, we confirmed 23 produced calves, suggesting a 12% (3/26) loss during gestation. Furthermore, 2 (20%) cows classified non-pregnant by ultrasonography in Oct. produced a calf demonstrating bison females may breed later than early September. Serum samples of breeding-age cows collected during the 2000 to 2007 roundups were also analyzed for serum P4 concentrations to assess herd-wide corpora lutea (CL) activity. Serum P4 during roundup was highly correlated (R ² = 0.89; P < 0.05) with calf production the following year, indicating fall estrus cycling with CL production as the likely major determinant of calf production. Body weights (BW) of 4- to 7-year old cows during roundup from 1999 to 2008 were highly correlated (R ² = 0.98; P < 0.05) to calf production the following year, indicated that the changes in calf production and fall estrus cycling were likely related to cow body weights. Calf production recovered to 56% and 67% in 2009 and 2010, respectively. However, historic records from 1956 to 2010 revealed calf production began declining around 1980. In conclusion, the decrease in calf production at the NBR appears to be due primarily to the number of breeding-age females that exhibit breeding and ovulatory activity during the breeding season, presumably due to cow body condition. The specific causes for these effects are not known but may be related to range resources.