In northern Australia, cattle graze pastures with a very low crude protein content and digestibility during the extended dry season, resulting in low live weight gain, or live weight loss. This poor performance is linked to an inadequate supply of N to the microbial population in the rumen and a low efficiency of microbial protein production. True protein supplements can be provided under these conditions to increase live weight gain but current prices make this uneconomical. To improve live weight gain of cattle grazing low quality dry season pastures across northern Australia, cheaper protein sources for cattle grazing in these scenarios is required. In this context, single cell protein sources, more specifically micro-algae, may be an alternate protein source to traditionally used supplements. Algae may become available as a ruminant feed source, either as a by-product of other industries or as an on-farm produced protein source. The objectives of this thesis were to investigate the composition of various algae species (Chapter 2), compare the effect of different algae species with a traditionally used protein supplement on rumen function and microbial protein production (Chapter 3) and feed intake and live weight gain (Chapter 4) in cattle and to examine the fatty acid profile in the rumen of cattle supplemented with various lipids (Chapter 5) or grazing different forages (Chapter 6).
Spirulina and Chlorella had the highest protein content of the algae species (675 and 580 g/kg DM, respectively). Spirulina had the highest in vitro protein degradability (67%), Schizochytrium had the highest lipid content (198 g/kg DM) and Dunaliella had the highest mineral content (720 g ash/kg DM). These four micro-algae spp. may have potential uses as supplements for ruminants when the production goal is to increase microbial protein production, alter the rumen fatty acid profile or to target a specific mineral imbalance, respectively.
In Chapter 3, supplementation of steers with Spirulina or Chlorella increased total dry matter intake (21.2 and 18.2 g DM/kg W.d, respectively), microbial protein production (0.82 and 0.87 g MCP/kg W.d, respectively), the efficiency of microbial protein production (90.7 and 105.9 g MCP/kg DOMI, respectively) and the fractional outflow rate of liquid from the rumen (7.7 and 4.9%/h, respectively) compared to unsupplemented steers fed a forage with a low crude protein content (24 g CP/kg DM, total DM intake = 12.6 g DM/kg W.d, MCP production = 0.28 g MCP/kg W.d, EMCP = 52.1 g MCP/kg DOMI). The responses to Spirulina and Chlorella in the diet were similar to the responses to cottonseed meal. In Chapter 4, the intake and live weight gain response to increasing amounts of Spirulina, cottonseed meal and a non-protein nitrogen (NPN) supplement were compared in steers fed a forage with a low crude protein content (33 g CP/kg DM). At low supplement intakes there was a greater live weight gain in steers fed cottonseed meal compared with Spirulina and the NPN supplement but at higher intakes equivalent to 5.6 and 4 g DM/kg W.d, for cottonseed meal and Spirulina, respectively, there was no difference between the two supplements. Higher allocations of NPN would not be possible due to toxicity issues. There was no difference in the effect of increasing supplement intake on total or hay intake for each of the three supplements. Chapters 3 and 4, indicate that Spirulina and Chlorella may be potential alternate protein sources to increase live weight gain of cattle fed low crude protein forages when offered at 4 g DM/kg W.d.
Throughout this thesis there were only small changes in the rumen fatty acid profile in response to various algae (Chapter 3) and lipid (Chapter 5) supplements or when steers grazed a range of different forages (Chapter 6). None of the supplements or forages had major effects on CLA c9, t11, the main CLA isomer, indicating it is unlikely that they would have any major differential effects on fat synthesis, with fat synthesis in animals fed low and high quality tropical forages more likely to be influenced by intake and stage of maturity of the animal. Furthermore, supplementation of cattle with protein supplements is likely to provide only small amounts of lipids to cattle which are unlikely to result in major changes in the rumen fatty acid profile which appear to be largely influenced by the fatty acid profile of the forages themselves and the extent of biohydrogenation of the lipid supplements. There was very little difference in the fatty acid profile of forages or the fractional outflow rate from the rumen of the wet season forages evaluated, which probably explains the lack of difference in fatty acid profile in the rumen of steers grazing the different forages, and suggests that it is the quality of forage, as determined by forage management, rather than any intrinsic forage property that determined fractional outflow rate and rumen fatty acid profile.