Kinetic aspects of calcium metabolism in forage fed sheep

Abstract

The kinetics of calcium metabolism in forage fed sheep has been investigated using ⁴⁵Calcium and compartmental analysis, using the SAAM program. Two physiological states, lactation and growth, were studied. During early lactation, calcium metabolism was determined in animals consuming fresh ryegrass-white clover (DMD: 70.6% and Ca: 5.48 mg/g DM) and greenfeed oats (DMD: 68.9% and Ca: 3.79 mg/g DM). The importance of protein status for calcium metabolism in those ewes consuming ryegrass-white clover swards was investigated by protein supplementation. In a further experiment net faecal endogenous loss of calcium was estimated in growing sheep consuming fresh or conserved forages, both contrasting in digestibility (DMD: 72.7 v 57.1%, respectively) and calcium content (Ca: 12.1 v 6.41 mg/g DM, respectively).

Trial one, undertaken during the first nine weeks of lactation, protein supplementation (100 g/d/head of protected casein per os.) increased milk production by up to 24%, gastrointestinal calcium absorption (mean ± s.e.m. 51.8 ± 15.3 v 75.4 ± 4.4 mg/d/kg LW for control and protein groups, respectively) and skeletal calcium accretion (41.7 ± 3.8 v 48.6 ± 5.3 mg/d/kg LW). Skeletal resorption was decreased (66.9 ± 14.4 v 59.9 ± 8.6 mg/d/kg LW) and the overall calcium balance increased (-25.2 ± 17.2 v -11.5 ± 4.2 mg/d/kg LW). In trial two, unprotected casein was infused (100 g/d/head abomasally) and milk production was balanced between control and supplemented groups offered ryegrass-white clover herbage (DMD: 78.1% and Ca: 6.47 mg/g DM). Protein supplementation significantly improved nitrogen balance in both trials and changed calcium parameters, similar in both, although not significantly. Possible reasons for this are discussed.

Calcium availability from oats was lower than that from ryegrass-white clover (17 v 19%, respectively); this was unexpected because of the lower calcium content of the oats. All availability values obtained were much lower than current ARC's.

Net faecal endogenous loss varied with dry matter intake (19.6 to 53.6 mg/d/kg LW) in wethers. When data from all three trials were combined, the best predictor of endogenous loss was faecal calcium output. Factors affecting this loss are discussed.