THE INFLUENCE OF UREA TREATMENT ON THE INTAKE OF WHEAT STRAW IN SHEEP

The aim of this experiment was to determine the factors responsible for the frequently negative effects on intake of straw treated with urea, a source of ammonia. Five forages were prepared from the same wheat straw: untreated straw (UNS) used a control; anhydrous ammonia treated straw (NH3S); straw treated with urea alone (US); straw treated with urea and ground soyabean (USS); and straw treated with urea, soyabean and molasses (USMS). They were distributed ad libitum in conjunction with 170 g/day of soyabean oil cake to 5 sheep over 5 different periods. The experimental design was a 5 x 5 latin square. Ureolysis was low, moderate and high with US, USS and USMS respectively. Ammonia treatment had the greatest effect on the digestibility of plant cell walls (+12.5 points), followed by the treatment with urea and soyabean (+10.7 points). The treatment with urea alone was not very effective (+6.6 points). The treatments had a clearcut effect on the level of dry matter (DM) intake: +13%, -32%, -2% and +27% (of which 14% for the straw) for NH3S, US, USS and USMS respectively. The intake of US and USS during the main meal after distribution was much lower than that of the other 3 straws. With US, there were a large number of secondary meals. The intake rate of US was very low, 1.78 g of DM/min, and that of USMS high, 3.09 g of DM/min, compared with 2.57 g on average for the other 3 straws. The other variables concerning intake and rumination are also presented. The reticulo-rumen contents were always lower with US than with the other straws. They were highest with NH3S. In contrast, with NH3S and US the turnover rates of the DM of the contents were the same (3.3%). The DM content, pH and volatile fatty acid contents of the reticulo-rumen contents were measured. With US and USS the ammonia content was high (596 and 455 mg/l respectively on daily average). The results suggest that urea treatment decreased straw intake because ureolysis was too low, owing to feed unpalatability and perhaps to high ammonia content in the rumen. It is unlikely that the animals' digestive capacities (rumen fill and content turnover rate) were in any way responsible. More work needs to be carried out on improving the methods of treatment to achieve maximum ureolysis with urea-treated straw so that its intake is higher than that of control samples.