The above picture indicates the relationship between size and daily gain in determining Feed Conversion Efficiency (FCE). The individual figures (MJ ME / kg live weight gain) relate to efficiency (the smaller, the better) at a particular gain for a specific size.
The following conclusions can be drawn:
- The faster an individual grows, the more efficient the growth.
- At the same growth rate, the smaller animal is more efficient.
- Individuals of varying size have to grow in proportion to their size in order to be equally efficient.
- Although individuals have to grow in proportion to their size in order to be equally efficient, they can’t eat in proportion to their size.
My overall conclusion is that grass / feed conversion efficiency is primarily determined by Relative Intake.
Grass is basically “digested” by microbes in the rumen. I doubt whether there are large metabolic differences between individuals on the same grazing / feed. The main confusing factor when measuring conversion efficiency as done in growth tests is the difference in maturity type (fatness) between breeds / individuals. That is why the South African FCE feedlot test favour lean genotypes. This is equally true for the current “advanced” RFI / NFI / NFE being promoted in Canada, the USA and Australia.
Prof. Victor Siller from the University of Monterrey in Mexico, who I met a few months ago, sent me a video presentation where RFI is measured and degree of fat accounted for. I still have a problem with this approach as I believe, from their own research, that age at puberty and cow fertility are still negatively affected. Another problem is the cost involved in measuring RFI.
Surely 12 Month Maturity and package assessment are much more appropriate (takes the greater whole into consideration) and cheaper. IF RFI was an appropriate measure of efficiency why has natural selection not favoured these genotypes rather than Relative Growth / Fatness?
I believe we should keep things simple and deduce from nature. Focus on detail leads to problems.