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Lactational Responses to Dietary Magnesium, Potassium, and Sodium during Winter in Florida^1,2,

Dairy Science Department, Institute of Food and Agricultural Sciences, University of Florida, Gainesville 32611

⁴ To whom requests for reprints should be made.

ABSTRACT

Forty-eight midlactation Holstein cows were used to evaluate dietary treatments arranged as a 4 x 2 x 2 factorial: .26, .38, .48, or .60% Mg, .24 or .62% Na, and 1.14 or 1.59% K. Supplemental Mg, K, and Na were supplied by feed-grade magnesium phosphate, potassium bicarbonate, or sodium chloride. All dietary treatments were equal in Ca and P. There were no effects of dietary Na or K on feed intake or milk production. Feed intakes were equal with .26, .38, and .48% Mg but declined 4.9% with .60% Mg. Milk yields responded curvilinearly to dietary Mg. Similarly, 4% FCM yields responded curvilinearly, increasing 7% with .48% Mg compared with .26% Mg then declining with .60% Mg. Milk fat percentages were unaffected by dietary Mg concentration, but milk fat yields responded curvilinearly. Milk protein percentages declined linearly as dietary Mg increased. Plasma Mg concentrations increased linearly from 2.52 to 2.68 mg/dl as dietary Mg increased. Renal fractional excretions of Ca responded curvilinearly as dietary Mg increased and decreased as dietary K increased. Results of this experiment suggested that current recommendations for dietary Mg do not maximize lactational performance. A companion laboratory experiment showed that feed-grade magnesium phosphate had less alkalizing capacity than two MgO sources.

¹ Florida Agricultural Experiment Station Journal Series Number 7748.

² Mention of commercial products does not constitute either endorsement or disapproval by the authors, Dairy Science Department, Institute of Food and Agricultural Sciences, or the University of Florida.

³ Dairy Business Group, Purina Mills, Inc., PO Box 66812, St. Louis, MO 63166-6812.

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