Wool growth in sheep fed diets based on wheat straw and protein supplements

Abstract

Wool growth of sheep fed wheat straw and various nitrogen (N) supplements was studied in two experiments lasting 16-20 weeks. In experiment 1, penned Crossbred wethers were fed wheat straw supplemented with urea (36 g kg-1 straw) and starch, or equivalent (N basis) amounts of formaldehyde-treated (F) casein (FCAS), rapeseed meal (RSM) or F-treated RSM or sunflower meal (FRSM and FSFM), all at the same total DM intake of c. 1240 g day-1. They produced from 7.9 to 13.9 g clean dry wool (CDW) day-1, and gained from 40 to 100 g day-1 for the urea and FCAS diets respectively. Mean digestibilities of organic matter (OM) and cell wall constituents were 0.53 and 0.42, with few differences between diets, while N digestibility of the FRSM diet (0.76) was significantly (P < 0.05) lower than all others (mean 0.89). During 1-6 h post-feeding, mean rumen pH and volatile fatty acid levels (6.43 and 55.2 mM respectively) were similar for all diets, whereas rumen ammonia levels were significantly (P < 0.01) higher with the urea and RSM supplements compared with the F-treated supplements. In experiment 2, penned Merino wethers fed 770 g DM day-1 of the urea-supplemented diet produced 5.0 g CDW day-1 and lost 50 g live weight day-1. In other sheep, fed at the same DM intake on wheat straw plus increasing levels (c. 70-350 g sheep-1 day-1) of an FRSM supplement, wool growth was quadratically related to supplement-N intake, with a maximum of 10.1 g CDW day-1 at a N intake of 15.6 g day-1. Digestibility of OM and N balance were significantly (P < 0.05) reduced at FRSM levels < 127 g sheep-1 day-1, while mean rumen ammonia levels during 6 h post-feeding remained < 70 mg N L-1 at FRSM levels < 183 g sheep-1 day-1. It was concluded that protein supplements resistant to ruminal degradation will substantially increase wool production in sheep fed straw diets at approximately maintenance levels.