The base document for nutrient requirements used by nutritionists is the 2001 Dairy NRC publication. A group of scientists appointed by the Committee on Animal Nutrition, National Research Council, periodically updates the available information. Significant new information in the current edition includes the following:
• A computer model to assist in diet evaluation.
• A summative equation approach to predict the energy content of feedstuffs.
• Metabolizable protein (MP) replaces the crude protein (CP) system.
• A discussion on amino acids.
• Mineral bioavailability factors for different classes of feeds and mineral supplements.
• A section on nutrition and the environment.
• An expanded discussion of carbohydrates.
One of the most important concepts defined in this publication is that feed nutrient values are not static, but change with level of feed intake and rate of passage. The Dairy NRC model was used to examine feed nutrient values for dairy cows producing 35 or 55 kg of milk per day. The same total mixed ration (TMR) was fed in this example. A 680-kg dairy cow producing 35 kg of milk per day had a predicted dry matter intake (DMI) of 23.6 kg. The net energy (NE)-l value for the TMR was 1.67 Mcal/ kg of dry matter (DM). Rumen degradable protein (RDP) was 9.9% of total DM. The cow producing 55 kg of milk had a predicted DMI of 30.2 kg. The TMR had an NE-l value of 1.58 Mcal/kg of DM. RDP was 9.6% of total DM. In this example, the same TMR had a 5% lower energy value when fed to the higher-producing cow.
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