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.
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