The shift to defining mineral absorption coefficients (AC) by feed class and type of mineral supplement was a step forward in the 2001 Dairy NRC. Previous NRC publications had assigned AC values by mineral rather than feed type. Calcium can be used as an example. The AC in the 1989 Dairy NRC was 0.38 for calcium and did not vary by source. The 2001 Dairy NRC uses an AC of 30% for forages and 60% for concentrate feeds. The AC value also varies from 30% to 95% from different mineral sources. A similar approach is used for other minerals.
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