GROWTH OF MOOSE CALVES CONCEIVED DURING THE FIRST VERSUS SECOND ESTRUS

Abstract

It has been hypothesized that a low bull:cow ratio can result in delayed or late breeding in some female moose (Alces alces). A consequence of late breeding is late born calves. It also has been speculated that late born calves grow faster and eventually attain a size similar to early born calves. We tested this accelerated growth hypothesis by breeding cow moose during their first or second estrus, and tracking the growth rates of their calves. We conducted the experiment over a 4 year period using 10 mature cow moose that produced 33 calves in 22 litters. Birth mass of calves conceived during the first and second estrus did not differ (P = 0.613) but mass of single calves was greater (P = 0.006) than twins regardless of date conceived. Body mass gained from birth through autumn (Oct( of calves born to cows bred during their first estrus was significantly (P = 0.0019) greater than calves conceived during the second estrus. However, by spring (May), mass gain was not significantly different (P=0.1368) between the two groups. We reject the hypothesis that second estrous calves exhibit accelerated growth during their first summer of life. Body mass of second estrous calves, however, increased at a faster rate than that of first estrous calves during winter (P = 0.0094), indicating that the potential for accelerated growth at least while on a high nutritional plane. By autumn as yearlings, mass of second estrous born calves was not significantly different (P = 0.125) than mass of first estrous calves, suggesting compensatory growth for second estrous calves during their second summer. There was no relationship (P = 0.1424) between April body mass of short yearlings and their gain in body mass over summer. We concluded that second estrous calves do not gain more mass by fall and consequently enter winter at a lower body mass. As a consequence, they are more likely to be susceptible to winter mortality, especially in deep snow years period management implications are discussed.