Variation in sire genetics is an irrelevant determinant of digestibility in supplemented crossbred sheep fed canola and lupins

The efficiency with which sheep produce meat and/or wool relies on a combination of available high quality nutrition and good genetics, hence the constant quest for sheep breed combinations that best utilise feeds to the maximum. High digestibility and nutrient retention of feed on offer are important indices of protein and energy available for wool fibre synthesis or muscle accretion in sheep. Pitchford et al. (1992) stated that both wool fibre number and diameter were strongly genetically determined. To our knowledge, it has not yet been established if sire genetics alone, influences the digestibility and nutrient retention of dietary energy and protein in supplemented crossbred sheep. Therefore, the objective of this study was to evaluate the influence of sire genetics, dietary protein supplement type, level of feeding, sex and their second order interactions on metabolisable energy and nitrogen digestibility in first cross progeny of Merino dams sired by 5 ram breeds. Weaner sheep (40) sired by 5 ram breeds (Poll Dorset, Coopworth, Texel, East Friesian and White Suffolk), were balanced for liveweight (30kg) and body condition score (3.0) on the average, before being randomly assigned to two supplementary feeds (canola or lupins) and fed at two levels (1% or 2.0% BW). The feeding trial lasted for six weeks including an initial adjustment period of 3wks and the final 1wk of faecal and urinary collection. All treatment groups received a daily allocation of an isocaloric and isonitrogenous diet comprising 0.5kg of barley, 0.1kg molasses-treated straw and 0.001kg vitamin-mineral mix at 10:00hrs. Each sheep had ad libitum access to clean, drinking water. Percentage digestibilities of metabolisable energy and nitrogen were computed and data subjected to a general linear models procedure (PROC GLM) of the Statistical Analysis System (SAS Institute 2007). The model included sire breed, supplement, feeding level, sex as main effects and their second order interactions. DM intake, faecal output, ME digestibility, nitrogen digestibility, body weight, and change in micron diameter were computed and significance established using orthogonal contrasts and Tukey's pairwise comparisons. Regardless of sire genetics, feeding level or gender, sheep supplemented with canola consumed 4.5% more feed (DMI 163.5 vs 149.2g day-1), voided 17% more faeces (51.08 vs 35.97g day-1), digested 8.5% more ME (52.23 vs 44.23%) and had 4% heavier liveweights (40 vs 36.9kg) than those supplemented with lupins. While lupins are the major legume grain fed to ruminants in Australia, their value as a source of undegraded protein is limited by their high rumen degradability and low methionine content (White et al., 2000, Rodehutscord et al., 1999). Canola meal on the other hand, is partially protected from degradation in the rumen with estimates of degradability ranging from 0.6 to 0.8 (Hill, 1991). These trends conform to our study's findings. Feeding supplements at 1% of body weight triggered higher ME (49.9% vs 46.5%) and N (64.9% vs 63.2%) digestibility responses than feeding at 2%. There was a tendency for females to eat more than males (161.8 versus 149.6g day-1DMI) and N digestibility was 2% higher in males (65%) than females (63%). Sire genetics x level of feeding interactions significantly influenced ME and N digestibility (P<0.05) whereby Coopworth-sired sheep supplemented at 1% of their body weight recorded the highest ME and N digestibility of 54% and 67% compared to 42% and 62% respectively, in their counterparts fed at 2% of body weight. There was a highly significant (P<0.01) effect of type of supplement x level of feeding interaction on wool fibre diameter because sheep fed canola supplements at 1% of body weight had finer wool (22.1 microns) than their 2%-fed counterparts (25.4 microns). Canola meal has been reported to contain more methionine (approximately 2 g/100 g CP; than lupins (0.4-1 .0 g/100 g CP; Hill, 1977) or rumen digesta (1 2-1 6 g/100 g amino acids; Wallace, 1994) and is therefore expected to provide a more appropriate source of amino acids for wool growth as demonstrated in our study and Masters and Mata (1996). Regression of wool fibre diameter on digestibility revealed that there was no correlation between the two (R2 = 0.0087-0.169). The postruminal delivery of nutrients in sheep supplemented with canola was more efficient than in lupin-supplemented sheep, hence their higher energy and protein digestibility and retention. Furthermore, variation due to sire genetics alone was insufficient in accounting for differences in digestibility and wool fibre diameter, but significantly interacted with type of supplement, level of feeding and sex. Finally, sire breed variation in digestibility is unlikely to be a useful predictor of genetic merit for wool fibre diameter in first cross sheep.