Effects of mild water restriction on nutrient digestion and protein metabolism in desert-adapted goats

Abstract

• Mildly restricting drinking water intake by ≤ 21 % does not reduce feed intake in desert-adapted goats. • Declining drinking water intake enhances rumen fermentation, but reduces microbial growth. • Lowering drinking water intake shifts nitrogen excretion from urine to faeces. • Effects of drinking water restriction differ depending on diet and animal factors. The aim of the present study was to determine the effects of mild drinking water restriction ( WR ) on feed intake, diet digestibility, rumen fermentation, and nitrogen ( N ) balance in a desert-adapted goat breed of Oman. Two feeding trials were conducted during the dry summer months (August-October) in 2013 and 2014. In each trial, nine adult male Batinah goats were exposed to two levels of mild WR (WR: 85 % (WR15) and 70 % (WR30) of individual ad libitum consumption; 100 % (WR0)) in a 3 × 3 Latin Square design. Drinking water intake, feed offered and refused, and excretion of urine and faeces were quantified and samples analysed for crude nutrient and fibre fractions. Moreover, rumen microbial protein synthesis was estimated from urinary purine derivatives excretion. In trial 2, rumen fermentation characteristics were additionally studied in three rumen-fistulated goats. Linear and quadratic effects of WR in each trial were assessed by mixed model analysis, whereas correlations between drinking water intake and response variables were determined across both trials. For WR30, drinking water intake was 79 % and 90 %, respectively, of the water intake of WR0 goats in trial 1 and WR15 goats in trial 2. Water restriction had no effect on feed intake, rumen microbial protein synthesis, and N balance in both trials. However, apparent total tract digestibilities of neutral and acid detergent fibre tended to decrease linearly ( p ≤ 0.081) with increasing WR level in trial 1, whereas those of dry matter and organic matter were lower ( p ≤ 0.028) for WR15 than for WR0 and WR30 in trial 2. Proportions of butyrate ( p ≤ 0.046) and isovalerate ( p = 0.087) in rumen fluid were greater and those of propionate ( p = 0.079) and valerate ( p = 0.062) lower for WR30 as compared to one or both of the other WR levels. Duodenal microbial N flow and efficiency of microbial protein synthesis did not differ between WR levels. Yet, across both trials, both variables correlated positively with drinking water intake. Similarly, declining drinking water intake increased the ratio between faecal and urine N excretion (r=−0.55) and the N concentration in faeces (r=−0.42), but reduced the fibre concentration (r=0.38) and the proportion of undigested feed N in faeces (r=0.49). Mild restriction of drinking water intake to ≥ 79 % of the ad libitum consumption, which is expected to occur more frequently with advancing climate change in the semi-arid tropics and subtropics, may impair feed nutrient use and thus performance of regionally adapted small ruminants, while improving the quality of their manure.