Calf:Cow Ratios
Merit A Closer Look

by Mike Thompson,
Wildlife Biologist
Fish, Wildlife & Parks
From the Pathfinder weekly column: Game Range Ramblings
February 5, 1998

 

Oddballs always seem to attract attention, and oddball calf:cow ratios are no exception. Nor should they be. It's hard to sustain an elk population without calves, and calf survival rate can be an early indicator of important trends in habitat quality and herd health.

During last month's helicopter survey of the Game Range, we only counted 12 calves for every 100 cows we observed. It was the lowest calf:cow ratio that FWP has recorded for the Black foot-Clearwater elk herd since intensive, nearly annual surveys began in 1988. This year's total count of 70 calves was as low as last year's count. Last year, half of the population wasn't even surveyed!

This year's low calf:cow ratio is truly an oddball. Normal ratios range from 27 to 37 calves per hundred cows. But, there's also an oddball at the opposite extreme in FWP's records of calf:cow ratios on the Game Range. In February 1993, FWP observed a ratio of 46 calves per hundred cows. This unusually high ratio stands as oddly apart from the norm as the low ratio we observed this winter.

Both extremes beg explanation. And, it may be argued that both resulted from extreme environmental events that affected elk winter range.

The cause of this winter's low calf:cow ratio can be traced back to last winter. One year ago, this year's calf crop was growing in the wombs of cows that had long forgotten what grass looks like. They were struggling in three to four feet of heavy, crusted snow, eating conifer nee dles and lichen, waiting for a spring that would come as late as winter had come early. Pregnant cows simply didn't consume enough nutrients to spare, and their fetal young suffered. Calves that survived to be born last June were apt to be late, light, and slow.

Under the circumstances, we should be grateful for those 70 calves that undoubtedly came from some of the herd's best stock. From the helicopter, these stubborn calves appeared healthy and normal. Wouldn't it be interesting if we could identify the cows that nourished their calves against all odds, and to understand how and why they succeeded when others could not? Had they secured premium habitats, and if so, where are these crucial sites?

At the opposite extreme, we were basking in the flow of an exceptionally high calf:cow ratio only five short years ago. At FWP's annual meeting in Seeley Lake last month, Jack Rich asked for an explanation of this oddball ratio we observed in 1993. I brought up the possibility of a beneficial effect from the wildfire of 1991, but promptly discounted it after considering the timing of events. Now, I think I may have withdrawn my fire theory too hastily.

Those 239 calves we counted in February 1993 were conceived in September 1991. On October 12, 1991, about half of the bunchgrass winter range burned in the Game Range wildfire. When the elk herd arrived on the winter range in November and December, they concentrated their feeding activities in the grassland portion that didn't burn, and shifted off the main Game Range when those resources ran out in January. The winter was unusually mild, so this brief shortage of dried bunchgrass forage probably caused the elk herd no great amount of stress.

The winter of 1991-1992 was so mild, in fact, that spring thaw began in February. This set the stage for an unusually early spring greenup, which was accel erated even further by the nutrients and heat-absorbing properties in the black ash bed. Judging from other research on the subject, the grass shoots that first emerged under these circumstances were probably as succulent and nutritious as they could ever become.

Meanwhile, tiny fetal calves were awakening to their most critical season of rapid growth and development. Their efforts were nourished within their mothers' bodies by an uninterrupted flow of supercharged fluids, fueled by foods cows ate on the burn. By June of 1992, it may have been raining elk calves around Seeley Lake, and those calves should have been larger and more likely to survive than normal. This may have caused the exceptionally high calf:cow ratio we observed from the helicopter in February 1993.

In my opinion, the early arrival of spring in 1992 was a critical factor. If spring had been late instead, fetal calves may have been too far along in their development to benefit as much from new green growth on the burn. As always, timing is everything. And, now, it's time to go.

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