The role of horsetails (Equisetaceae) in the nutrition of northern-breeding geese.

The nutrient content and balance of the horsetail Equisetum fluviatile was measured in samples taken from southern James Bay during early spring, summer and fall, and related to the nutrient requirements of geese at these times. Young emergent stems contained in excess of 20% protein of well balanced amino acid composition, apart from deficiencies of cystine and methionine. The ash fraction contained high levels of calcium, magnesium, potassium and phosphorus, which would satisfy the mineral needs of breeding and growing geese. The aerial parts of E. fluviatile did not contain much energy as neutral fats and soluble carbohydrate. The growth pattern of E. fluviatile, with young aerial stems arising throughout the summer, would provide a source of succulent, high-protein food. This could be used by geese migrating to the breeding grounds, during egg formation, to facilitate growth of goslings, and in the reacquisition of body protein lost during reproduction. The nutritive value of E. fluviatile was not high during the fall.