Abstract/Summary

As ruminants, cattle are marvellous bioreactors that, through symbiotic rumen fermentation, convert cellulosic plant biomass and other organic materials inedible to humans into high-quality animal proteins for human nutrition. Nevertheless, the conversion is of course not 100% efficient, and so varying quantities of waste products such as carbon dioxide (CO2), methane (CH4), and reactive nitrogenous compounds are emitted. As such, producers of ruminant livestock must strive to maximise output for each unit of input, both to enhance enterprise profitability and to minimise the environmental impacts of dairy and meat production. A key metric of this system efficiency is feed conversion efficiency (FCE), which for milk production is usually defined as energy-corrected milk divided by feed dry matter intake (DMI) and for meat production is live weight gain divided by feed DMI. FCE per se also has a genetic component, which can be measured by residual feed intake (RFI). RFI is defined as the actual intake minus the feed intake expected to meet requirements for milk production, growth, reproduction and maintenance (Koch et al., 1963). FCE has been widely used in beef production, as well as in pork and poultry production, to monitor the efficiency of feed utilization for growth. The dairy industry also recognizes the importance of the metric in management systems, but in addition to milk yield, there is also a need to account for body tissue loss and gain in calculating the efficiency of a lactating dairy cow (VandeHaar, 1998). Maximising the output of saleable product per unit of resource input is a standard principle of all manufacturing industries that relate directly to profitability. Another way of stating this relationship is that producers must minimise their unit cost of product and optimise their total unit output (Colman et al., 2011). Relative to the reduction of greenhouse gases and contaminants of water, the simple concept is that the more carbon and nitrogen (N) in feedstuffs captured in the product, the less carbon and N are available for conversion into waste products (e.g. CO2, CH4 or urea N). By this principle, increasing milk or meat output from the same feed input requires changes in digestibility or postabsorptive nutrient metabolism with the result that less greenhouse gases and other waste products are produced per unit of milk or meat. The same principles apply to phosphorus and other nutrients that may become pollutants when they escape the animal through feaces or urine. This chapter will focus on the efficiency of milk production by dairy cattle related to nutrition and geneticsfocussing on how improving FCE can decrease the greenhouse gas burden of milk production and how FCE can be improved.