Chemical Composition of Douglas-Fir Foliage on Mule Deer Winter Range
In the interior of British Columbia, Douglas-fir litterfall is a major source of mule deer winter food. Dawson et al. (1990) found that preference for Douglas-fir foliage was correlated with tree diameter. This study set out to identify the underlying ultimate factors of selection so that wildlife managers might have a wider range of forage enhancement options on mule deer winter range. Such options might include, for example, planting, thinning, or fertilizing trees to improve palatability.
Samples of Douglas-fir foliage were collected from trees at Knife Creek and Big Lake, and analysed for minerals, tannins and in vitro digestible dry matter (IVDDM). Phosphorus was the only mineral correlated with preference in three of four study sites. It also correlated with dbh on two other sites where preference was not known. Douglas-fir samples were found to be phosphorus-deficient, so perhaps small differences in concentration are important to deer. Tannins, which are digestibility-reducing compounds, were not well correlated with preference. No preference data were collected on sites where IVDDM of Douglas-fir was measured. Digestibility did not seem to affect selection of Douglas-fir foliage, as suggested by the lack of correlation between IVDDM and dbh. Our results, however, must be interpreted with care because the study had design limitations. Future studies could be improved with an increased sample size, the testing of a wider variety of chemicals, and the use of foliage that has a wider range of chemical concentrations.
The nutrient content of Douglas-fir is important because it is a large part of deer winter diet in the Cariboo Forest Region. The Douglas-fir foliage sampled meets minimum ruminant requirements for nitrogen (crude protein), potassium, magnesium, iron, zinc, and calcium/phosphorus ratio. Sulphur, sodium and, in some cases, calcium and copper concentrations are lower than required. Manganese is 2-10 times greater than the suggested forage level. Deficiencies may be supplemented by other plants during winter or by plants eaten in other seasons.
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