Feeds for dairy cows are often valued according to their price per megajoule (MJ) of Metabolisable Energy (ME), as energy intake is usually limiting with cows during early lactation. ME is a measure of the energy available to cows, after losses from undigested feed, urine and methane are accounted for.
ME has some limitations as it takes no account of the different products of degradation, digestion and absorption of different nutrients.
- Volatile fatty acids (VFA) are produced in different ratios by degradation of different carbohydrates in the rumen. Sugars and starches produce mainly propionate, and fibre mainly acetate.
- Acetate is used as an energy source in the body, and is important in the synthesis of fats, hence why high fibre diets increase milk fat percent. Propionate is a major substrate for glucose production in the liver, which is important for production of proteins in the udder, hence why milk protein percent increases. Butyrate is also produced from sugars and starches, is converted in the rumen wall to beta-hydroxybutyrate (BHOB), which is used for energy production in tissues.
- Unsaturated oils/fats (lipids) are toxic to some rumen microbes, so are converted to saturated fats that are absorbed in the small intestine. Triglycerides (3 fatty acids (FA) attached to glycerol) are the major lipids in non-forage feeds, and glycolipids (2 FA + glycerol) are the principal lipids in forages. Glycerol is cleaved off being an energy source by the rumen microbes. Ruminants are unique as they absorb fatty acids, not triglycerides, in the small intestine, so protected fats need to be free fatty acids. Absorbed fats bypass the liver, and provide energy in tissues or fat production in the udder.
ME values of feeds indicate the potential energy supplied by them, which may not be achieved, if partially degraded (in the rumen) or digested and absorbed (in the abomasum and small intestine).
Net Energy (NE), not ME, accounts for energy used to digest and process feeds (heat of digestion), which is highest with poor quality forages and lowest with fats. For example, the ME of protected fatty acid at 37 MJ/kg DM is 2.8x that of wheat at 13 MJ/kg DM, whereas the NE of FA at 30 MJ/kg DM is 3.6x wheat at 8.3 MJ/kg DM.
Space, does not allow consideration of differing metabolic effects of absorbed nutrients on metabolism and physiology, but it should be apparent, that just comparing feeds based on ME may not always be appropriate.
As featured in NZ Dairy Farmer