Effect of water intake on the nitrogen balance of sheep fed a low or a medium protein diet

Abstract

Mutton merino wethers (n = 16) fed either a low nitrogen (LN, with a crude protein [CP] content= 5.4%) or a medium nitrogen (MN, with CP = 10.3%) diet were subjected to two water treatments; either ad libitum or half ad libitum access to water, in a two-period, two-treatment cross-over design. Sheep were adapted to the diets for at least three weeks before the start of the trial, and were kept on each water treatment for one week. A digestibility trial was carried out during the last five days of that week, and concomitant jugular blood samples were taken daily. The group fed the LN diet consumed 820 ± 128, 44.6 ± 6.9 and 2828 ± 524 g/day organic matter (OM), CP and water respectively, when allowed free access to water. Restricting water intake to 1463 ± 330 g/day reduced the intake of OM and CP to 697 ± 183 and 37.9 ± 9.9 g/day respectively. In the group fed the MN diet, reducing the water intake from 2937 ± 372 to 1406 ± 301 g/day reduced the OM and CP intakes from 936 ± 64 and 96.6 ± 6.6 to 665 ± 92 and 68.6 ± 9.4 g/day respectively. As a result, urine output decreased from 944 ± 327 and 772 ± 150 ml/day to 505 ± 60 and 509 ± 276 ml/day for sheep fed the LN or MN diets respectively. Water restriction in the MN group decreased the amount of nitrogen retained from 45.9 to 9.4 g/day, largely as a result of the decreased intake of nitrogen (about 30 g/day). Although the intake of OM also decreased proportionately, the amount lost in the faeces remained the same, suggesting that OM digestibility was decreased (from 68 to 52%). In the LN group, restricting the water intake improved nitrogen retention from 1.1 to 9.2 g/day, despite the depressed intake of feed CP. This was due to a large decrease in the amount of nitrogen lost via the faeces. There was also a concomitant increase in OM digestibility (54 to 71 %). Although the amount of nitrogen lost via the faeces was found to be relatively constant (about 4.5 gN/day), this was reduced when the amount of water lost via this route declined below 500 g/day. In sheep fed the MN diet, restricting water intake reduced CP intake. In sheep fed the LN diet, the same restriction resulted in a reduced nitrogen excretion via the faeces, rather than a reduced CP intake. Nitrogen loss via the urine was not affected by restricting water intake in the sheep fed the LN diet, in contrast to the MN-fed group, in which CP intake was drastically reduced, while not materially affecting the amount of nitrogen excreted in the urine. Restricting water intake halved urine production in both groups of sheep. The amount of water lost via the faeces was only halved in the sheep fed the LN diet, while those fed the MN diet showed no change. Neither diet nor water treatment affected the glomerular filtration rate (GFR). When sheep were fed a diet containing adequate CP, restricting water intake severely reduced the amount of nitrogen retained. However, when sheep were fed a diet low in CP, the same restriction appeared to increase the amount of nitrogen retained. The effect of infrequent watering may therefore appear to ameliorate the often low-protein grazing associated with arid areas.