Effect of dietary cation-anion difference on mineral balance, serum osteocalcin concentration and growth in weanling horses

Sixteen Quarter Horse weanlings were used in a split-plot design experiment to determine the effects of dietary cation-anion difference (DCAD) on mineral metabolism, osteocalcin concentration and variables of growth. Horses were blocked by age and sex and then randomly allotted to two treatment groups (High or Low). Experimental diets with a DCAD of +325 (High, H) and -52 (Low, L) were formed by supplementing diet H with sodium bicarbonate and diet L with calcium chloride. The DCAD was calculated as meg (Na + K) - (Cl + S)/ kg of diet DM. The 25-week trial consisted of three 72 h collection periods at 150, 240 and 330 d of age during which a complete urine collection was taken. Multiple Fecal grab samples were collected to represent every 2 h post-feeding during each collection period. Venous blood sample!; were taken for analysis of serum osteocalcin concentration. Weekly measurements were taken to monitor skeletal growth. Representative samples of feed, feces and urine were analyzed for mineral content and apparent mineral balances were calculated by difference. Although osteocalcin concentration did not differ significantly between diets H and L at 150 and 330 d of age, it was higher (P<.05) at d 240 in horses consuming diet H versus diet L. No difference (P>.05) was detected between treatments in any skeletal growth variable measured. Phosphorus balance and digestibility did not differ significantly between treatments across d of age. However, digestibility of P was lower (P<.05) at d 240 and 330 compared to d 150 in both treatments. Horses consuming diet L experienced an increased (P<.05) urinary and a decreased (P<.05) fecal excretion of Ca compared to horses consuming diet H. Apparent daily Ca balance was higher (P<.05) in horses consuming diet L than those on diet H at 240 and 330 d of age. Furthermore, Ca digestibility decreased significantly between d 150 and d 240 and 330 in horses consuming diet H. Horses consuming diet L experienced no difference (P>.05) in Ca digestibility across d of age. These findings suggest that horses consuming a low DCAD diet are able to compensate for the increased urinary excretion of Ca by enhancing intestinal Ca absorption and thereby increasing daily Ca balance and digestibility.