Phosphorus Utilization in Sheep

showed a lower appetite, which affected feed intake and P absorption and reten- tion. In this case, there was an overlap of the effects of C. punctata beyond the lesions in the intestinal mucosal lining, which is the indirect action reducing feed intake. The chronic infection simulates animals at pasture, and the severity of the process resulted in lower mineral retention (4.31 g P/day) compared with healthy calves. With the acute infection the difference in retention between healthy and infected calves was 1.71 g P/day.

In the fi eld, endoparasitosis is generally associated with more than one species of parasite, and it is clear that they interfere mainly in mineral metabo- lism, thereby increasing P requirements. This fact should be taken into consid- eration in herds reared extensively. Phosphorus is one of the most studied minerals in ruminant nutrition worldwide, but important questions about its requirements need to be answered. Phosphorus is a non-renewable element and is responsible for environmental contamination and therefore its use should be optimized.

true P absorption of rock phosphates are lower than for dicalcium phosphate or superphosphate. Arrington et al. (1963) showed that P from dicalcium phosphate is absorbed and retained in larger amount than that of rock or soft phosphate.

Phosphorus from some rock phosphates may also be highly available, but the utilization in diets is limited due to fl uorine content.

The reason for the relative unavailability of some P sources in general has not been identifi ed. Whether the diffi culty lies in the problem of solubility and absorption or in the ability of the animal body to utilize what is absorbed is not known (Ammerman et al., 1957). Some phosphates are capable of forming polymers of various lengths (Hendricks, 1944) and if this occurs in the intestines, absorption might be impaired. Variations in true availability among similar P sources obtained with the same methodology could be due to the origin and material processing.

Effects of dietary phosphorus levels on phosphorus metabolism

Quantitative aspects of P metabolism in sheep fed different levels of P have been considered using balance studies with the isotope dilution technique (Vitti et al., 2000, 2008; Dias et al., 2007). Experiments were carried out with 5-month-old (23 kg) Brazilian Santa Inês breed male sheep (exp. 1). The treat- ments consisted of the basal diet supplemented with different amounts of dicalcium phosphate (0, 8.3, 16.6 and 25 g) to provide 0, 1.5, 3.0 and 4.5 g P/animal/day (Dias et al., 2007). Another experiment (exp. 2) used 8-month- old (33 kg) Santa Inês male sheep (Vitti et al., 2008). The protocol used was the same as previously described. The treatments consisted of a basal diet supplemented with different amounts of dicalcium phosphate to provide 0, 2, 4, and 6 g P/animal/day.

According to the standard classifi cation scheme (NRC, 2007), the P require- ment for sheep of a LW between 20 and 30 kg is 4.0–5.1 g/animal/day. There- fore, P levels were classifi ed as low (< 3.0 g), adequate (3.1–5.7 g) and in excess (> 5.8 g). Plasma P was in the normal range (4–6 mg/dl; NRC, 2007) and addi- tional P had no effect on P in plasma and urine. Although no values were observed below 4 mg P/dl, the use of inorganic P in plasma as an indicator of P status in ruminants is the subject of divergent opinions among researchers, as mentioned earlier. Plasma P is infl uenced by many factors, such as intake, bone metabolism, starvation and time of collection (Silva Filho et al., 2000). Urine P was low, representing less than 2% of P intake.

Phosphorus intake had a linear relationship with faeces P (Eqns 4.2 and 4.3). A similar relationship was also reported by Vitti et al. (2000) and Dias et al. (2006) working with lambs and goats. Total P excreted in faeces repre- sented an average of about 75% of P intake in both experiments. Values of faecal loss as high 95% of total P intake have been reported (Salviano and Vitti, 1996).

P_faeces = 0.71 P_intake + 0.88 (exp. 1, r²= 0.88, n = 12, P< 0.001) (4.2) P_faeces = 0.86 P_intake− 0.08 (exp. 2, r²= 0.92, n = 24, P< 0.001) (4.3)

The endogenous faecal loss of P increased with increased P intake. For 5-month-old sheep (Table 4.3), the endogenous loss represented 65% of P intake and the relationship between those variables was:

Endogenous P = 0.31 P_intake + 0.78 (r²= 0.64, n = 12, P < 0.05) (4.4) This equation indicates that the endogenous loss at zero P intake is 0.78 g/day or about 18 mg P/kg LW/day. This minimum value represents the inevitable loss that would occur if sheep were fed to meet their maintenance requirement. This value is in agreement with the recommendation of NRC (1985) (20 mg P/kg LW/day) and it is higher than the values indicated by ARC (1994) (9–12 mg P/kg LW/day). Assuming the mean coeffi cient of absorption of 0.65 (Table 4.3), the animal must consume 1.28 g total P/day (0.78/0.65) to meet maintenance requirement.

Table 4.3. Effect of dietary P concentrations on P metabolism in sheep. Experiment 1 used 5-month-old sheep (23 kg) and experiment 2 used 8-month-old sheep (33 kg).

Experiment 1 (Dias et al., 2007)

Parameters (g/day)

Treatments (g P/animal/day as dicalcium phosphate)

0 1.5 3.0 4.5

P intake 0.91^d 2.55^c 4.43^b 6.59^a

P plasma (mg/dl) 6.80^a 8.89^a 9.05^a 7.29^a

P in faeces 1.40^d 2.90^c 4.10^b 5.50^a

P urine 0.003^a 0.004^a 0.006^a 0.005^a

Endogenous P in faeces 0.70^b 2.00^a 2.70^a 2.90^a

Dietary P in faeces 0.70^b 0.80^b 1.40^b 2.60^a

P absorbed from diet 0.20^c 1.70^b 3.00^a 3.90^a

P availability (coeffi cient) (%) 0.23^b 0.67^a 0.68^a 0.60^a

Retained –0.50^b –0.30^b 0.30^a,b 1.10^a

Experiment 2 (Vitti et al., 2008)

Treatments (g P/animal/day as dicalcium phosphate)

Parameters (g/day) 0 2.0 4.0 6.0

P intake 1.47^d 3.53^c 5.69^b 7.47^a

P plasma (mg/dl) 6.66^a 7.91^a 8.78^a 9.02^a

P in faeces 1.10^d 3.18^c 4.40^b 6.55^a

P urine 0.0034^a 0.006^a 0.025^a 0.125^a

Endogenous P in faeces 0.60^c 1.60^b 2.41^a 2.43^a

Dietary P in faeces 0.50^c 1.58^b 1.99^b 4.12^a

P absorbed from diet 0.97^c 1.95^b 3.52^a 3.36^a

P availability (coeffi cient) (%) 0.66 0.55 0.62 0.45

Retained 0.37^b 0.44^b 1.11^a 1.12^a

Within rows, different superscripts denote signifi cantly different (P < 0.05).

The endogenous loss in exp. 2 represented 41% of P intake (Table 4.3) and endogenous P was also related to intake:

Endogenous P = 0.31 P_intake + 0.36 (r²= 0.62, n = 24, P< 0.05) (4.5) The calculated endogenous loss was 0.36 g P/day or 12 mg/kg LW/day for 8-month-old sheep. The coeffi cient of absorption was 0.57; therefore, the cor- responding P intake for maintenance is 0.63 g P/day. This indicated that age affected the minimum endogenous loss and that younger sheep needed a higher P intake for maintenance. More information on endogenous P excretion and true P absorption is required to calculate P requirements accurately, which would mitigate P pollution from animal agriculture.