SUMMARYPhosphorus requirements for maintenance and growth were assessed by giving to growing lambs and non-pregnant ewes diets low in P and to growing lambs a diet containing the quantity of P recommended by the Agricultural Research Council (1980).Seven 14-week-old lambs were given ad libitum a pelleted diet containing 1·88 (LP) or 2·75 (NP) g P/kg D. M. for 13 weeks. There was no effect of P intake on feed intake or growth during the first 8 weeks when the P intake of the LP groups was 1·07 of Agricultural Research Council (1980) requirements. During the last 5 weeks the P intake of the LP group was only 0·87 of Agricultural Research Council (1980) requirements and growth and feed intake were higher in the male but not female lambs in the NP group. From the 3rd week the plasma concentration of P was significantly lower (P <0·001) in the LP group. Mineralization of selected bones was less in the LP group.After 14 weeks on the diet, balance trials with 32 P were carried out on the lambs from each dietary group. P intakes ranged from 1 to 3 g/day. An additional balance trial was undertaken with eight 2-year-old female sheep consuming a hay diet (1·5 g P/day).The endogenous faecal excretion of P by the growing lambs on the LP diet and ewes was 12·2 ± 0·8 and 12·7 ± 1·00 mg/kg live weight/day respectively, very close to the value (12 mg/kg live weight/day) adopted by the Agricultural Research Council (1980). The absorbability of the P in the concentrate and hay diets was 0.71 ±0·267 and 0·74 ± 0·0218 respectively, the former agreeing with and the latter higher than the corresponding value for Agricultural Research Council (1980).

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.

The isotope dilution technique was used to measure endogenous faecal excretion and absorption of dietary Ca and P by growing lambs. Groups of four lambs were given 0-8 or 1-2 kg/day of a pelleted barley and soya-bean diet containing 4-2 g P/kg D.M. and supplemented to contain 1-79, 3-52 or 4-67 g Ca/kg D.M.At the lowest dietary concentration of Ca, absorption of Ca was insufficient to meet the Ca required for deposition in the newly formed bone matrix of the skeleton; retention was 0-39, 1-16 and 1-12 g/day with increasing dietary Ca concentration. The proportion of dietary Ca absorbed decreased (P < 0-001) with increasing Ca intake, ranging from 0-85 to 0-44. Endogenous faecal excretion of Ca was independent of the absorption of dietary Ca, but increased (P < 0-05) from 17-2 at the lower to 21-1 mg/kg live weight/day at the higher D.M. intake. Plasma concentration of Ca was higher for the female (2-51) than the male (2-33 mmol/1) lambs and increased linearly (P < 0-001) with dietary concentration of Ca from 2-31 to 2-52 mmol/1.The retention of P increased with Ca and D.M. intake. On the other hand, the proportion of dietary P absorbed (0-85) was independent of Ca intake and hence of P retention. Increases in P retention were reflected by decreases in urinary P, but not in endogenous faecal excretion. Endogenous faecal excretion of P increased with D.M. or P intake from 17-9 at the lower to 33-5 mg/kg live weight/day at the higher intake.The results are discussed in relation to dietary allowances of Ca and the mechanism of P homoeostasis. TNTROTYTTPTTONmaintenance requirements consist predominantly of unabsorbed secretions of minerals into the digestive Two major difficulties exist in the factorial tract, any factor which alters the rate of secretion approach to the prediction of the dietary require-or the absorbability of the minerals in the secretions ments of ruminants for Ca and P . These are the will alter requirements. Many factors, including the choice of value for the coefficient of absorption of physical nature and quantity of feed (Bailey & dietary Ca and the maintenance requirements of Balch, 1961) are known to increase the volume of P. The difficulty with Ca is that ruminants absorb secretions and Braithwaite (1982,1983) has recently Ca according to needs (Agricultural Research shown experimentally that faecal excretion of endo-Council, 1980). Thus the wide divergence in the genous Ca varies directly with feed intake. Of published values for the proportion of dietary Ca greater concern is whether salivary P, the main absorbed (see Agricultural Research Council, 1980) source of secretion of P, is determined by the can be attributed to the fact that in many of these quantity of P absorbed surplus to requirements or studies Ca intake was rarely limiting and, for a by the actual quantity of P absorbed. The latter given net requirement, the proportion will vary would imply that maintenance requirements are inversely with Ca intake.not constant but vary with the requirements for In the factorial approach t...

Thirty-eight mature Scottish Blackface sheep were mated at pasture and 32 brought indoors when 3-4 weeks pregnant. Eight were killed at this stage as controls (CI). The remaining 24 were offered a basal hay ration (200 g/head/day) supplemented with a semi-purified diet containing 84 g crude protein/kg and either 1-5 (Px), 2-8 (P 2 ) or 9-3 (P 3 ) g P/kg dry matter. The rate of feeding of the supplement allowed a mean daily P intake (g/day) in groups P x , P 2 and P 3 of 1-1, 1-6 and 4-7 and 1-6, 2-5 and 7-9 during weeks 4-15 and 16-21 of pregnancy, respectively. Six well-nourished sheep, fed conventionally, provided control ewes and lambs.Balance trials were conducted on four sheep from each of groups P 1( P 2 and P 3 during weeks 9-10, 14-15 and 19-20 of pregnancy. All sheep were slaughtered at parturition and selected bones of ewes and lambs removed for histological and chemical examinations. Whole body fat, nitrogen, Ca and P contents of the ewes and mineral content of the lambs were determined.The faecal P excretion of the P x sheep ranged from 10-8 to 19-1 mg/kg body weight/ day; urinary P excretion ranged from 0-7 to 2-9 mg/kg body weight/day in early and mid-pregnancy and from 1-0 to 8-1 mg/kg body weight/day in late pregnancy. It is suggested that on this evidence the minimum net faecal and urinary endogenous P loss can be no greater than about 13-5 mg/kg body weight/day. Carcass analysis and bone histology and chemical composition confirmed that the P requirement of pregnant ewes is much smaller than has been generally assumed.It is suggested that for a 50-kg ewe carrying twins the P requirement increases from an average of 1-1 g/day during the first to about 2-5 g/day during the last month of pregnancy.

SUMMARYOf a group of 24 pregnant Scottish Blackface sheep 12 were killed at the outset and the remainder offered, ad libitum, a poor quality hay. At the end of pregnancy seven more were killed and the remainder offered, during lactation, 3 kg/head/day of a semipurified diet of 64% dry-matter (D.M.) digestibility, and containing 18% crude protein (CP), 1·17% Ca and 0·82% P in the D.M. Milk production was determined at weekly intervals and milk samples obtained at these times for estimation of Ca and P concentrations. These sheep were killed 42 days post-partum. The shorn empty bodies were analysed for fat, protein, Ca and P contents.The feeding of the poor quality hay caused losses of 76, 37, 15 and 15% of the body contents of fat, protein, Ca and P, respectively, during pregnancy. Despite maintaining a moderate milk yield (1·5 kg/day) the ewes were in positive Ca and P balance during early lactation and had restored the skeleton to its initial mineral content by 42 days post-partum. The rate of absorption of Ca was calculated to have been at least 115 mg/ kg body weight (BW) which is double other estimates in the literature for sheep at this stage of lactation.These differences are discussed in relation to mechanisms for adaptation of Ca metabolism which have been proposed for other species.