EFFECTS OF SUMMER-AUTUMN NUTRITION AND PARTURITION DATE ON
REPRODUCTION AND SURVIVAL OF ELK
John G. Cook
Project Leader, National Council for Air and Stream Improvement, 1401 Gekeler Lane, La
Grande, OR 97850, phone (541-962-6536), fax (541-962-6504),
email cookjg@eou.edu,
Bruce K. Johnson
Project Leader, Oregon Department of Fish and Wildlife
1401 Gekeler Lane, La Grande, OR 97850, phone (541-962-6556), fax (541-962-6504), email
johnsobd@eou.edu,
Rachel C. Cook
Wildlife Biologist, National Council for Air and Stream Improvement,
1401 Gekeler Lane, La Grande, OR 97850, phone (541-963-9275), fax (541) 962-6504, email
rcook@eou.edu,
Robert A. Riggs
Wildlife and Range Scientist, Boise Building Solutions
1907 Jackson, La Grande, OR 97850, phone (541-962-2046), fax (541-962-2046),
email RobertRiggs@BoiseBuilding.com
Tim DelCurto
Associate Professor, Eastern Oregon Agricultural Research Center, Union, OR 97883, phone
(541-562-5129), fax (541-562-5348), email Tim.DelCurto@oregonstate.edu,
Larry D. Bryant
Range Scientist, USDA Forest Service, Washington, D.C. 20250, phone (202-205-0850), fax
(202-205-1096), email lbryant01@fs.fed.us, and
Larry L. Irwin
Principal Research Scientist, National Council for Air and Stream Improvement, Stevensville,
MT 59870, phone (406-777-7215), fax (406-777-7213), email llirwin@bitterroot.net,
Declining elk herds in the northwestern U.S. has sparked interest in those factors that may
contribute to the declines. Two factors, among others, have been hypothesized to contribute to
the declines: (1) delayed breeding-delayed birthing leading to reduced calf survival due to few
mature bulls in the population, and (2) inadequate quality and quantity of forage to maintain
productive, large populations via density-dependence/nutritional mechanisms. We evaluated
potential contributions of both factors in an experimental setting using captive elk from 1995-98.
Early (Sep) and late (Oct) breeding dates, selected to represent ranges expected due to variation
in mature bull abundance (<3 weeks), and three levels of summer-autumn nutrition, selected to
represent ranges in forage quality available to elk in summer-autumn on inland NW summer
ranges, were applied as treatments in a 2 X 3 factorial design. Earlier breeding date significantly
affected calf weights by the subsequent winter, but the magnitude of the difference was
insufficient to significantly affect over-winter calf survival. In contrast, the nutrition levels we
implemented had marked effects on calf growth and their ability to survive in winter, yearling
growth, fat accretion in cows, probability of pregnancy in adult and yearling cows, and overwinter survival probability of adult cows. Our data suggest that low mature-bull:cow ratios
probably have not contributed appreciably to declines in elk herds of the region. But our finding
that just moderate nutritional deficiencies (particularly digestible energy deficiencies) in
summer-autumn have large effects on reproductive performance leaves open the possibility that
forage adequacy indeed contributes to the declines. Thus, our data indicate a need for greater
understanding of nutrition’s influence on population dynamics and how this influence varies
across space and time, particularly in response to various habitat management options. To
enhance future research, we compiled animal- and vegetation-based guidelines for assessing
nutritional influences on elk populations.