Effects of concentrate starch level and free-choice provision of straw on performance, feeding behaviour and feed sorting of fattening lambs

The importance of forage in small ruminants' behaviour is well known, but very few experimental studies have investigated the feeding behaviour of lambs offered ad libitum and as a free choice provision of concentrate and wheat straw. This experiment was conducted to investigate the effect of concentrate starch level on growth performance and behaviour, preference toward straw and rumen fermentation in lambs offered ad libitum forage and concentrate. Twenty-four Iranian Arabic male lambs (mean +/- standard deviation; 18.8 +/- 2 kg of BW) were used in a completely randomized design with 3 treatments (8 lambs per treatment) for a period of 84 days. Experimental treatments were included 3 levels of starch (27.1%, 35.4% and 43.6% of dietary dry matter (DM)) and wheat straw was offered as a free-choice provision. Total DMI was not different among treatments; however, intakes of straw (P < 0.01), peNDF (P = 0.003), water (P = 0.05), and ME (P = 0.005) linearly increased with increasing level of starch in the concentrates. The increased dietary starch concentration linearly increased total gain (P = 0.02), and average daily gain (P = 0.01), but linearly decreased feed conversion ratio (P = 0.02). Increasing starch in the concentrates linearly increased sorting for straw particles retained on the 19-mm sieve and tended to increase sorting for particles retained on the 8-mm sieve. In contrast, sorting against the fine particles was observed with increasing starch content of the concentrates. Eating time decreased linearly as the amount of starch in the diets increased. Time spent ruminating per kg DM and NDF intake linearly increased with increasing starch in the diet. Standing time increased linearly (P = 0.04) as starch content of the diets increased. Lambs fed low starch diet had higher pH values than medium starch (at 2, 4, and 6 h post-feeding) and high starch (at 0, 2, 4 and 6 h post-feeding) diets. These findings suggest that under free-choice provision of straw and concentrate lambs are able to adjust their sorting behaviour in favor of physically effective NDF and more forage intake with increasing concentrate starch level, which could be an adaptive response to attenuate low rumen pH.