Effect of diets low in phosphorus and high in dietary calcium on phosphatemia and phosphorus and calcium levels in rumen contents of sheep

Durand, M.; Bertier, B.; Hannequart, G.; Guéguen, L.

Reproduction, Nutrition, Developpement 22(5): 865-879


Four semi-purified diets with different calcium (Ca) and phosphorus (P) contents were given to adult sheep at a level of 1 kg daily plus 100 g of straw. The phosphorus was supplied as dicalcium phosphate, almost insoluble in rumen fluid. In the preliminary period, all the animals (16) were fed the test diet (NCa-NP) containing 8.4 g of Ca and 3.3. g of P. In the experimental period (table 3) two trials were carried out. In trial 1, two groups of 5 sheep each were given for 5 weeks a diet low in P (1 g/kg) and either low (2 g/kg) (diet BCa-BP) or high (10 g/kg) (diet HtCa-BP) in calcium. In trial 2, which lasted for 19 weeks, the HtCa-BP diet was compared to a high Ca (10 g/kg) normal P (3.3 g/kg) diet (diet HtCa-NP); two groups of 2 sheep each were fed both these diets every 3 hours, and one group (HtCa-BP) was fed twice daily. The concentration of ultrafilterable inorganic Ca (CaiU) in the rumen contents was related to the Ca level of the diet (tables 4, 5). However, individual variations were high and CaiU concentrations were negatively correlated to rumen pH (fig. 7). When diets adequate in P were fed, ruminal concentrations of ultrafilterable P (PiU) were rather high (400-490 mg/l), showing the importance of endogenous P supply in the rumen. With an adequate P supply, a high Ca intake had no effect on either plasma or ruminal PiU. When low P diets were fed, there was a steady decrease in plasma Pi from 6 to 3.5 mg P/100 ml and in ruminal PiU from 500 mg/l to values lower than 300 mg/l up to 5-6 experimental weeks. These decreases were greatest with high Ca intake (HtCa-BP diet) (figs. 2, 3), some animals being markedly affected (ruminal PiU values lower than 20 mg/l). However, the decreases in ruminal PiU were less pronounced with frequent feeding than with feeding twice a day. Considering all the results obtained, ruminal PiU concentrations were positively correlated with plasma Pi concentrations (r = + 0.77; n = 75) (fig. 8). Therefore, high Ca intake can enhance the response to P deficiency and reduce further plasma Pi concentrations and levels of available P in the rumen. Microbial P requirements may not be satisfied in such conditions, which would reduce ruminal microbial digestion in some animals.