SEASONAL-VARIATION IN INTAKE AND DIGESTION OF A HIGH-ROUGHAGE DIET BYMUSKOXEN

We studied changes in body weight, intake, retention time and apparent digestibility of a supplemented brome-alfalfa hay by mature muskoxen held near Saskatoon, Saskatchewan during two seasonal cycles. Body wei ghts and voluntary intake were monitored during 16 trials from late Ma rch 1990 to December 1991, in four annually breeding females and in th ree hysterectomized females. Retention time of the hay and apparent di gestibility of the diet were estimated in winter and summer 1991 from fecal excretion of chromium (Cr) after a pulse dose of Cr-mordanted ha y, and from forage/fecal concentrations of chromic oxide, respectively , in six non-breeding muskoxen. Mean (+/-SE) body weights of breeding females were greatest in February and March (248 +/- 5 kg) and lowest following calving in late May or early June (216 +/- 6 kg), while mean weight of hysterectomized females declined little from a peak of 228 +/- 21 kg in February-March to a low of 213 +/- 21 kg in July. Mean dr y matter intake of breeding females was highest in Jul and August (62 +/- 3.6 g d-1 kg-0.75) then declined Slowly to its lowest point in Mar ch and April (41 +/- 0.7 g d-1 kg-0.75). Dry matter intake of hysterec tomized cows was less clearly seasonal but averaged 42 +/- 1.7 g d-1 k g-0-75 in late winter and 50 +/- 4.9 g d-1 kg-0-75 in mid-summer. Mean retention time of hay, calculated using a noncompartmental model, dec lined significantly from 114 +/- 4 h in March to 95 +/- 4 h in July. A pparent organic matter digestibility decreased significantly from 73.8 +/- 0.8% in winter to 61.7 +/- 1.3 % in summer. Compared with other r uminants, muskoxen are grazers exceptionally well-suited to digesting graminoid forage, with relatively low maintenance requirements and pro longed retention times. Most of the seasonal changes in weight and int ake of breeding females could be ascribed to reproductive cost, but th ere also appeared to be seasonal effects on intake, retention time and body weight in non-breeding muskoxen.