Animal and dietary variation in the absorption and metabolism of phosphorus by sheep

Abstract

SUMMARYIn Expt 1 the efficiency of absorption of phosphorus in feedingstuffs was compared within four sets of 18-month-old, chimaera-derived triplets; 12 feedingstuffs were evaluated in four balance trials with 32P. Seven diets consisted of a P-rich ingredient mixed with a low-P basic diet and five of hay or pelleted dried grass or lucerne. The P ingredients tested were the protein-rich meals, rape, soya-bean, maize gluten, fishmeal and rice bran, and the cereals, barley and wheat.Sets of triplets absorbed dietary P with different (P < 0·001) efficiencies; the mean values were 0·72, 0·63, 0·76 and 0·79. The availability of P differed (P < 0·001) between diets, highest values being seen with the fishmeal (0·80), barley (0·78) and wheat diets (0·78) and the lowest with rice bran (0·63) and a perennial ryegrass hay (0·64). There was no evidence for an age effect on efficiency of absorption.Endogenous faecal excretion was positively related to intake (P < 0·001) and negatively related to efficiency of absorption of dietary P. Plasma concentration was positively related (P < 0·001) to both intake and, between sets, to the efficiency of absorption.Significant urinary excretion of P generally occurred when the efficiency of absorption was greater than 0·70. Two members of a set which absorbed P with high efficiency and excreted low volumes of urine died from urethral obstruction caused by calculi.In Expt 2 an estimate of the variation in the concentration of P in plasma of 48 ewes on a barley-based diet was assessed and the values for three of the chimaera sets fell within the top quartile.Safety factors were calculated from the animal and plant variations in the efficiency of absorption of dietary P and used to determine dietary allowances for different classes of sheep.

II. Quantitative aspects of phosphorus absorptionin ruminants

“…The constant true absorption coefficient values advocated [26,28] appear to be too low. However, the remaining variations are probably due to feedstuffs or diets [6,29,30], differences in phosphorus digestive availability [31] and also differences among species and physiological state.…”

“…Improvement of our knowledge of the mineral content of feedstuff [35] and of phosphorus quality [36] is the two keys for improved supplementation [31] and pollution management [37]. Despite its importance in the calculation of phosphorus supply, few experimental findings on phosphorus availability in ruminants are available.…”

“…A true absorption coefficient value of 0.60 would be suitable for diets with large amounts of hay whereas 0.70 would suit better to silage-based diets. More than 0.70 of the phosphorus in cereals is absorbed [31] and the values reported for cereal byproducts are also high: 0.70 in corn gluten meal [31] and 0.90 in wheat bran [42]. For this reason, the main criticism of the current systems is not the level recommended (0.50, 0.55, 0.60 or 0.70) but may concern the constancy of the recommended value.…”

I. A bibliographic database for quantitative analysisof phosphorus flow in ruminants

“…On this type of diet phosphorus repletion after nitrogen and phosphorus deficiency was also analysed Quantitative aspects of phosphorus metabolism 257 [18]. In sheep, 100%-forage diets were given in order to determine the availability of phosphorus in forages [38,39], maintenance phosphorus requirements [40,41] and the phosphorus metabolism [29,[42][43][44][45][46][47][48][49]. [50] used 100% chopped hay diet for the determination of net intestinal exchange of ions.…”

“…Pure concentrate diets were also given to lambs for the evaluation of various phosphorus sources [51,54], in another study of phosphorus metabolism [55,56], for the determination of the availability of phosphorus in different feedstuffs [38,57], and for studies of phosphorus excretion [45,58] and phosphorus metabolism [59]. In calves, 100%-concentrate diets were used for studying the influence of diet structure on phosphorus excretion [58,60] as well as the phosphorus metabolism [21].…”