Abstract:
Rapeseed meal (RSM), sunflower meal (SFM) and formaldehyde-treated (F) forms of these were compared with urea- and formaldehyde-treated casein (FCAS) as N supplements for sheep fed pelleted diets containing c. 80% oat straw. Sufficient supplement was added to bring the N content of each diet to c. 1.7%, and starch was added to the urea and FCAS diets to make all supplements equal in metabolizable energy content. Based on an assumed fractional outflow rate from the rumen of 0.06 h-1, the ruminal degradability of dietary N varied from a mean of 0.42 for diets containing formaldehydetreated supplements to 0.74 for non-treated supplements. The diets were fed to Crossbred wethers weighing c. 40 kg in a production experiment lasting 12 weeks, and in a metabolism experiment. In the production experiment, penned sheep fed the urea diet ate 1300 g DM day-1, gained 30 g Day-1 and grew 4.5 g clean dry wool (CDW) day-1 . Each of the protein supplements increased DM intake (mean 1860g day-1 P < 0-0l), liveweight gain (mean 148 g day -I, P < 0-001) and wool growth (mean 10.5 g CDW day-1; P < 0.01). Intake and liveweight gain were higher, and wool growth rate lower, on the SFM than on the RSM diet, while formaldehyde treatment of both meals stimulated intake, gain and wool growth although these differences were not always significant. Efficiency of wool growth, expressed as g CDW per g N digested, was least for the urea and SFM diets (mean 0.30), higher for the FCAS, RSM and FSFM diets (mean 0.40) and highest for the FRSM diet (0.48, P < 0.05). Mean digestibilities of organic matter (OM) and cell wall constituents for all diets were 0.47 and 0.44 respectively, with few significant differences between diets. Apparent N digestibility varied from 0.41 to 0.61 for the FRSM and FCAS diets respectively, while estimated true N digestibility varied from 0.72 to 0.88 for the same two diets. Nitrogen balance for the protein-supplemented diets (mean 2.68 g day-1) was higher (P < 0.001) than for the urea diet (-3.77 g day-1). During 6 h post-feeding, the mean rumen pH (6.50) and volatile fatty acid levels (60.9 mM ) showed few differences between diets, whereas rumen ammonia concentrations were significantly reduced by formaldehyde treatment of the protein supplements. It was concluded that RSM and SFM were effective supplements for sheep offered straw, with little difference between them except for wool growth. Formaldehyde treatment to confer resistance to ruminal degradation reduced N digestibility, but increased the efficiency of utilization of digested N for wool growth.