A comparative study on the excretion of urinary metabolites in goats and sheep to evaluate spot sampling applied to protein nutrition trials

Abstract

The main objective of this study was to establish a protocol to validate urine spot samples to estimate N excretion and microbial synthesis in goat and sheep; and to study factors that affect daily creatinine and purine derivatives (PD) urinary excretion. Also a performance trial was carried out to compare goat and sheep slaughtered after different feedlot periods. Twelve Boer goats (20.6 kg ± 3.4 initial BW) and 12 Dorper sheep (18.4 kg ± 2.3 initial BW), all 4-mo-old, males, were used. Eight animals (4 goats and 4 sheep) were randomly allocated to be slaughtered at 28, 56, and 84 d in feedlot. The experiment was conducted in a completely randomized design in a 2 × 3 factorial scheme, in which the factors were both species and the 3 feedlot periods. Diet consisted of 50% sorghum silage and 50% concentrate on a DM basis. Nutrient intake was higher (P < 0.01) for sheep than goats. Apparent digestibility of nutrients was similar (P > 0.05) in both species. Sheep had greater (P < 0.01) ADG and final BW than goats. Fat deposition and fat:muscle ratio was higher (P < 0.01) in sheep carcasses. Sheep had higher N urinary (P = 0.02) excretion and N retention (g/d; P < 0.01) than goats. Urinary N excretion increased linearly (P < 0.01) in response to feedlot period. However, feedlot did not affect (P = 0.20) N retention, but linearly reduced the relationship between N retained and ingested (P = 0.04) or apparently digested (P < 0.01). Microbial efficiency (P > 0.05) did not differ between species. Creatinine excretion (C mg/d; P < 0.01) was higher in sheep than goats. Purine derivatives (Ŷ) were related closely with OM intake (Ŷ = 0.013±0.0007X; r2 = 94). A difference (P < 0.01) was found between the allometric model for creatinine excretion (Ŷ) and muscle weight (X) for both species, and the following equations were obtained: Ŷ = 89.04(±31.44)X0.9797(±0.16) for goats and Ŷ = 109.8(±47.50)X0.8002(±0.20) for sheep. Creatinine concentration was greater during nocturnal than diurnal periods, with lower diurnal fluctuations. Sampling time did not affect (P = 0.27) the PD:C ratio. The urea (U):C ratio was higher (P < 0.01) in sheep than goats, and was also higher (P < 0.01) during diurnal than nocturnal sampling periods. Our results suggest that it is necessary to take 2 and 3 spot urine samples after feeding to estimate N compounds excretions in goats and sheep, respectively.