The following information was presented by Adam Lock, of Michigan State University, and Mike Van Amburgh, of Cornell University, at the Western Canadian Dairy Seminar last March.
Q: How can nutrition improve milk protein yields?
A: Of all constituents, protein typically has the highest value within milk payment schemes, sometimes receiving a 50 percent higher premium compared with milk fat. This has led to considerable interest in increasing milk protein yields. In contrast with milk fat, the impact of nutrition on milk protein content is poorly defined. In general, milk protein content tends to increase with increases in energy intake from carbohydrates, but decrease when relatively high levels of fatty acids are included in the diet. Dietary protein supplements typically increase milk protein secretion but have variable effects on milk protein content.
The general consensus in the literature is that increases in milk protein content can be realized by replacing grass silage with corn or whole crop silage, and greater use of high starch concentrates, and to a lesser extent with protein supplements. However, the scope to alter milk protein content is rather limited and much lower than that of milk fat. In most cases, a significant increase in the protein to fat ratio is only realized through feeding diets stimulating a reduction in milk fat content which in itself is often counter-productive.
The limitation to milk protein synthesis tends to follow a priority of nutrient supply. The primary factor is metabolizable energy supply through effects on insulin, insulin like growth factor–1 and other energy signaling mechanisms (e.g. Mackle et al., 2000) associated with enhanced microbial yield through increased ruminal carbohydrate fermentation. Integrated in the energy signaling are the presence of amino acids and mechanisms like mTOR that potentially modulate protein synthesis depending on not only the availability of energy but the availability of required essential amino acids (e.g. Rius et al. 2010).
From a practical and environmental point of view, to improve the efficiency of use of feed protein, formulation models and feeding management of lactating cattle should focus on reducing the excess protein in the diet. Any protein not required by the cow is excreted in the urine as urinary urea, a consequence of urea nitrogen recycling and the removal by the kidney of any urea not recognized by the animal as necessary for rumen function. Thus, the objective should be to reduce protein intake to the level necessary to maintain adequate rumen fermentation and microbial yield, and reduce the quantity of urea nitrogen excreted in the urine. This approach assumes that the nutritionist can evaluate the cattle and their diet adequately to determine what is first limiting, metabolizable energy (ME) or protein (MP) and then make the appropriate changes. If ME is first limiting, then reducing the excess crude protein, if done correctly should reduce urinary urea excretion and modestly improve overall efficiency of use of absorbed protein – simply by reducing the excess.
Several studies have been conducted that demonstrate the ability to decrease the CP content of the diet, increasing N efficiency and maintaining milk yield. For example, Cyriac et al. (2008) altered the 2001 Dairy NRC predicted RDP content of the diet from 11.3 to 7.6 percent of dry mater while simultaneously decreasing the CP content of the four diets fed to lactating dairy cattle. The RUP supply was held constant while the predicted RDP balance ranged from 301 to -574 g/d with corresponding CP levels of 18.4 to 13.6 percent, respectively. Overall dry matter intake and milk yield were not different when CP was reduced to 15.2 percent in this experiment, and efficiency increased from 27.7 to 35.5 percent. At the lowest level of dietary CP, 13.6 percent, dry matter intake and milk yield were significantly decreased and milk protein yield was also reduced suggesting that both rumen N and MP supply were not met and rumen fermentation was reduced, thus causing a decrease in intake.
Recent data from Cornell University (Recktenwald, 2007; Hofherr, 2010; Higgs et al., 2012) demonstrate that high producing lactating cattle can maintain adequate milk and milk protein yield on relatively low protein diets (14 to 15.5 percent DM) provided that adequate rumen carbohydrate fermentation is achieved and the balance between rumen ammonia and MP supply is properly formulated and evaluated. In studies by Recktenwald (2007) and Hofherr (2010) high producing cattle were able to achieve 42 to 52 kg of milk yield per day on diets ranging from 14.2 to 15.5 percent with high quality forages. The key factor was forage quality followed by proper characterization of the cattle characteristics and feed ingredient chemistry. In these studies, milk urea N (MUN) averaged approximately 8 mg/dL with a range of 5.5 to 10 mg/dL and a MUN less than 6.5 resulted in rumen ammonia measurements less than required to maintain adequate fiber digestion. The sum of the available data points indicates that there are efficiencies to be achieved by reducing the excess protein in the diet independent of increases in protein synthesis and milk excretion.