Effects of bull age on conception dates and pregnancy rates of cow elk

Productivity of cows in many Rocky Mountain elk (Cerous elaphus nelsoni) populations of northeast Oregon has declined over the last 30 years. Numbers of mature bulls declined concurrently, suggesting a potential link that accounts for declining productivity. We evaluated the influence of bull age on conception dates and pregnancy rates of cow elk within a 78-km(2) enclosure on the Starkey Experimental Forest and Range in northeast Oregon from 1989 to 1993. We allowed a single cohort of bulls to mature from 1 1/2 to 5 1/2 years and function as principal herd sires. Subsequent male offspring were reduced in numbers through hunting and trapping. We estimated conception dates, pregnancy rates, body condition, age, and lactation status of cows killed in December. Conception dates occurred earlier as bull age increased (P = 0.0001) and were significantly different between bulls less than or equal to 2 years and greater than or equal to 3 years of age. The rut became more synchronous and shortened from 71 days (n = 26) when breeding was by yearling bulls to 41 days (n = 33) when 5-year-old bulls were the principal sires. Pregnancy rates increased from 89 to 97% as bull age increased, but not significantly (P = 0.62). Cow body condition was highest (P = 0.004) in 1989 when breeding was by yearling bulls. To enhance herd productivity we recommend that elk hunting seasons be designed so that older bulls (greater than or equal to 3 yr) are retained in the population.