Differential responses to an oral urea load test in small ruminants: species and breed effects

Abstract

An experiment was conducted to determine the differential responses to an oral urea load test (OULT) in 1–2-year-old sheep and goats. Six non-bred ewes from each breed [Suffolk (SFK), Gulf Coast Native (GCN), Katahdin (KAT), and St. Croix (SCX)] and six non-bred does [Boer Goat Cross (BGC)] were individually penned and fed a concentrate-base diet at 120% of NRC (1985) requirement for maintenance along with bermudagrass hay ad libitum for 2 weeks. At the end of the 2-week-period, half of the animals from each group were fitted with jugular vein catheters, and blood was collected at 15 min intervals for 360 min. At the 45 min mark, the OULT (0.4 g urea/kg BW) was administered via a stomach tube. Ruminal samples were collected before and after the OULT was administered. The experiment was repeated with the remaining animals on the following day. Blood was analyzed for plasma ammonia-N, urea-N, glucose, insulin, and l-carnitine. Ruminal fluid was collected via stomach tube and analyzed for pH and ammonia-N. Comparisons were made for each group of animals to test for breed effect, and all sheep data were pooled to compare with goats and to test for species effect. After OULT administration, ruminal pH increased from 6.51 to 7.05 ( P<0.0001), and there was a seven-fold increase in ruminal ammonia-N 30 min post-OULT ( P<0.0001; 16.2 versus 92.5 mmol/l). Plasma urea-N and ammonia-N levels increased over time in response to the OULT ( P<0.0001), but there were no breed or species effects, breed×time, or species×time interactions ( P>0.10). A breed×time ( P<0.0001) and species×time interactions ( P<0.0001) as well as a species effect ( P=0.05) were observed for glucose in response to the OULT with GCN plasma glucose increasing the most. Likewise, there was a breed effect ( P<0.01) and a trend ( P=0.14) toward a breed×time interaction for insulin, although there was no species effect ( P>0.10) or species×time interaction ( P>0.10). There were no breed-related differences ( P>0.10) in plasma l-carnitine. The data suggest that breeds and species can respond differently to an OULT and when conducting or interpreting experiments using this type of test, breed and species should be considered.