Proper growth of the skeleton is vital to the healthy development of young horses.
Impediments to normal development of the skeleton can seriously impair the horse’s ability to perform to its maximum genetic potential. Nutritional factors have been linked with ‘developmental orthopedic disease’ in young horses (Tee- ter et al., 1967; Cunha, 1991). Although several nutritional factors have been associated with causing the disease, P defi ciency has been shown to be critical for the development and integrity of the skeleton in growing horses (Hintz and Schryver, 1972; Hintz et al., 1976). In spite of its importance in skeletal develop- ment, other factors need to be considered when trying to achieve the correct levels of P in food sources. Studies have concluded that exercise of the horse also infl uences the mineralization of bones and bone quality. Moreover, P over- supplementation can cause undesirable imbalances among minerals (Honoré and Uhlinger, 1991) and can be expensive (Furtado, 1996).
Common horse feedstuffs that are relatively high in P include cereal grains, oilseed meals (cottonseed meal and soybean meal) and some alternative feeds such as sugarcane yeast. However, most of the organic phosphate in feeds is in the form of phytate, which is relatively unavailable for absorption in non-ruminants.
Several studies have investigated the digestion and metabolism of P in horses (e.g. Schryver et al., 1971; Kichura et al., 1983), in which P bioavailability ranged from 30 to 45%. Hintz (1983) reported 32% and 40% P bioavailability in maize grain and oat grain, respectively. Phosphorus bioavailability can also be
infl uenced by P intake, age of the animals and Ca:P ratio (Schryver et al., 1971);
however, studies that quantify the effect are scarce.
According to Georgievskii (1982), P bioavailability can be determined by P adioisotope studies. Furtado et al. (2000a) evaluated the effect of different P sources on P metabolism in growing horses by determining their biological P availabilities. The authors evaluated Tapira rock phosphate, Patos rock phosphate,
VI
194.98 VU –
VIT – 16.69
Digestive tract
Vaa – 121.02 Vaf – 10.46
V0+R2
V0+R 122.25
VOT2 94.37 V0+D 94.56 Vf
6.23
Ve0– 6.08 VeT– 1.81 V0+R1
27.88
VeD1 8..97
VeD2
122.51 VO+T
27.95
VOT1 27.88
VTe–
26.69
VTe+
55.83
Soft tissue
Vo+
188.93 V0–
100.45 0.17
VF 80.19 VFD 73.96
VaT 131.48
94.37
Plasma
B o n e
Fig. 5.1. Phosphorus (P) distribution model for growing pigs fed a diet containing Tapira phosphate and a daily intake of 194 mg P/kg LW/day. The abbreviations are as follows: VI, P intake; VF, total faecal P excretion; Vf, endogenous faecal P; VFD, faecal P of dietary origin;
Vu, urinary P excretion; Vaa, absorbed P of dietary origin; VIT, endogenous P origin input to gastrointestinal tract (GIT); Vaf, endogenous P reabsorbed into the GIT; VaT, total absorbed P;
V0+, P accretion in the bone; V0−, reabsorbed P in the bone; VO+R2, P from bone recycled to bone and soft tissues; Ve0−, reabsorbed P from the bone into the GIT; VO+D, P from VaT accre- tion to the bone; VTe+, P accretion in the soft tissues; VTe−, reabsorbed P from the soft tissues;
V0+R1, P from the soft tissues recycled into bone and soft tissues; VO+R, total P recycled into bone and soft tissues; VeT−, reabsorbed P from the soft tissues into the GIT; VO+T, P from VaT to accretion in the soft tissues; VeD1, P from VaT into the GIT; VeD2, P from VaT into bone and soft tissues; V0T1, P from V0+R accretion into the soft tissues; V0T2, P from V0+R accretion into bone (Lopes et al., 2001).
dicalcium phosphate and bonemeal added to a basal diet to supply 22 g P/
animal/day. The animals were injected with 30 MBq 32P/animal and specifi c activities of plasma, faeces and urine were determined and faecal endogenous loss and true P absorption were calculated. They reported that P intake, excre- tion, plasma concentration and retention were unaffected by phosphate sources.
Reported P availability values were 25.2, 33.9, 31.7 and 29.4% for Tapira rock phosphate, Patos rock phosphate, dicalcium phosphate and bonemeal, respec- tively, which were not signifi cantly different from the commonly used source (dicalcium phosphate).
In a separate experiment, Furtado et al. (2000b) studied the effects of differ- ent P levels in the diet of growing horses on the endogenous faecal loss and P availability. They used three treatment levels (no P supplementation and dical- cium phosphate supplementation to give 15, 20 and 25 g P/day). Faecal endog- enous loss and true P absorption were calculated, based on specifi c activities of the radiophosphorus in the plasma, faeces and urine, for each horse in the experiment. A linear relationship between P intake and P excretion was estab- lished. About 73% of dietary P intake was excreted in faeces, confi rming that the main route of P excretion in horses was through faeces. There was also a linear relationship between absorbed P, retained P and dietary P intake. Urine P and faecal endogenous loss were reported to be unaffected by P levels.
Quadros (2006) studied the effect of Ca on P metabolism, using the isotope dilution technique, in growing equines. They used three levels of dietary Ca intake (0.15, 0.45 and 0.75% of diet), with same level of P (0.23% of diet), and did not fi nd signifi cant effects on P intake, plasma P, P in faeces, endogenous P in faeces, absorbed P and retained P. They concluded that P level in the diets met the requirement of the horses in the experiment and P metabolism was not affected within a Ca:P range of 0.65:1 to 3.2:1.
Another study on P metabolism was carried out using the isotope dilution technique to determine the P bioavailability of feeds in Brazil, using 12-month- old horses (Oliveira et al., 2008). Five diets were formulated to contain approxi- mately equivalent levels of crude protein and digestible energy and to supply the NRC (1989) recommended level of at least 22 g P/horse/day. All fi ve diets con- tained 40% Bermuda coastal hay plus 60% concentrate. The concentrates were maize + cottonseed meal (C1), maize grain + soybean meal (C2), maize + sugarcane yeast (C3), oat + cottonseed meal (C4) and oat + soybean meal (C5). Although all P diets resulted in a positive P retention, P intake was lowest in horses fed diet C3 (79.7 mg/kg LW). Absolute values of P concentrations in plasma, urine and faeces were similar in all treatments as well as endogenous P loss. Phosphorus bioavailability values were 50.8, 41.0, 43.5, 51.0 and 57.7% for diets from C1 to C5, respectively, and were signifi cantly different between diets C2 and C3 and the other diets. Bioavailability values of all dietary treatments exceeded NRC (1989) recommendations of 35% true P absorption in diets not supplemented with inorganic P. The results of this study indicate that inorganic P supplementation may not be required for growing yearlings fed common Brazilian feeds, and, taking into account cost and risk of environmental P contamination, the issue needs serious consideration and fur- ther confi rmation.