Influence of calcium (0.6 or 1.2%) and phosphorus (0.3 or 0.6%) content and ratio on the formation of urolithogenic compounds in the urine of Boer-cross goats fed high-concentrate diets

Abstract

The population of meat goats in the U.S. increased dramatically between 2002 and 2007. With this increase has come an increase in the number of cases of obstructive urolithiasis being seen by veterinarians. Since this condition is often associated with imbalances in dietary Ca and P, the goal of this project was to investigate the influence of concentration and ratio of Ca and P in diets for meat goats on the formation of urolithogenic precursors. Using 8 Boer-cross, wether goats in replicated Latin squares, 4 diets containing either 0.3 or 0.6% P, 0.6 or 1.2% Ca, 0 or 2% added salt, and Ca:P ratios of 1:1 or 2:1 were offered (0.3% P, 2:1; 0.6% P, 1:1; 0.6% P, 2:1; and 0.6% P, 2:1 + salt). Serum, urine, and feces were collected in conjunction with water and feed intake measurements. All treatment groups had similar dry matter intakes (DMI; 838–887 g/d, P = 0.19) and N retention rates (6.2–7.0 g/d, P = 0.54). Water consumption per unit DMI was similar for goats eating both Ca:P = 2:1 diets without 2% added salt (2.06 g/g DMI and 1.89 g/g DMI for 0.6% P and 0.3% P, respectively). Water consumption was also similar for goats receiving the Ca:P = 1:1 (2.50 g water/g DMI) and 2% added salt diet (2.79 g water/g DMI); and, these levels were higher than those for the lower salt, 2:1 diets ( P = 0.0002). Serum Ca, PO 4, Mg, K, Na, Cl, HCO 3, and anion gap all fell within normal ranges throughout the trial, as did serum urea N and glucose. Fecal DM was 10% lower in goats consuming 0.6% P, 1:1 than when other diets were offered (32% vs. 42%, P < 0.0001), possibly the result of changes in Na and water absorption in the gastrointestinal tract caused by high P and Ca:P imbalance. Goats consuming the 0.3% P, 2:1 diet had similar urinary crystal density scores (2.47 out of 3) to goats receiving the 0.6% P, 1:1 diet with Ca:P = 0.81 (2.31 out of 3). The 0.6% P, 2:1 diets without and with 2% added salt also had similar, but lower crystal density scores (1.50 for 0.6% P, 2:1 and 1.06 for 0.6% P, 2:1 + salt; P = .002). Goats with high crystal density scores had higher urinary P, Mg, or both. Our results suggested that the formation of urolithogenic compounds is highly complex and is the result of an interrelationship between multiple minerals in the diet, not only Ca and P. Mineral concentrations may interact with mineral imbalance to impact overall mineral and water absorption from the intestines. We also demonstrated that urolithogenic precursor crystals may easily be observed using light microscopy and suggested that the crystal precursors may provide a useful predictor of which goats may be prone to developing clinical urolithiasis.