Abstract:

Some environmental factors affecting the gross efficiency of crude protein (GECP) were studied using 2824 records of 501 lactating Holstein cows. The effects of some independent variables on the GECP were tested using a general linear model analysis. The GECP was significantly affected by parity and pregnancy stage and linearly related to dry matter intake, milk yield, milk fat, protein and lactose percentages, protein and lactose yields, dietary levels of net energy for lactation (NEL), crude protein (CP), ruminally undegradable protein (RUP), and ether extract (EE). Quadratic relation to the GECP with dietary levels of NEL, CP, RUP, and EE were also significant. This study showed that high producing animals, especially high protein yielding cows are more efficient users of dietary CP. Dietary requirements of NEL, RUP, and EE to maximize the GECP were estimated as 1.61 Mcal/kg, 7.14%, and 5.42%, respectively. In comparison to NRC estimations, it seems that more NEL, less CP, and more RUP are needed to maximize the protein efficiency of lactating dairy cows.

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