Growth and carcass characteristics of prime lambs fed diets containing urea, lupins or canola meal as a crude protein source

Abstract

Lupin grain typically provides the main source of protein in most lamb finisher diets in Western Australia. The aim of this work was to determine whether alternative sources of nitrogen could provide a less expensive means of achieving the desired rate of liveweight gain and carcass characteristics. Fifty-one crossbred lambs were finished using 3 different sources of crude protein. These sources were either: (i) a control diet consisting of lupins, oats, barley and cereal chaff in which lupins provided 49% of the nitrogen (lupin diet); (ii) a diet consisting of a combination of cereal grain and urea (urea diet); or (iii) a diet in which lupins were partially replaced with expeller canola meal (canola diet). Diets were isonitrogenous (14.4% crude protein) and isoenergetic (10.5 MJ ME/kg DM). Lambs were gradually introduced to the diets over 2 weeks before being fed the diets ad libitum for 5 weeks.

Lambs fed the canola diet performed the best in terms of feed intake and liveweight gain while those fed the urea diet performed the worst. Respective growth rates for sheep fed the canola, lupin and urea diets were 272, 233 and 180 g/day, respectively, with significant (P<0.05) differences only between the canola and urea diets. Equivalent dry matter intakes for the 3 groups were 1660, 1570 and 1380 g/day respectively, with sheep fed the urea diets being significantly (P<0.05) lower than the other two. Acidosis was considered to be a major cause of the poor performance of sheep fed the urea diet, with 3 sheep from this treatment showing inappetance and markedly reduced growth rates ('shy feeders').

Lambs finished on all 3 diets produced acceptable carcasses. Carcass weights followed the trend of growth rates, but there were no significant differences between diets in carcass weight, dressing percentage, GR tissue depth, C site fat depth, eye muscle area, muscle pH or meat colour. At the price of the feeds used in this trial, the canola meal diet was the cheapest feed per kilogram of liveweight gain despite being the most costly diet per tonne of feed.