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Apparent and True Digestibility of Magnesium in Mature Cows of Five Breeds and Their Crosses¹

Texas A&M University², College Station 77843

Abstract

Sixty dry, nonpregnant, mature cows of five breeds (Angus, A; Brahman, B; Hereford, He; Holstein, Ho and Jersey, J) and their F₁ crosses (AB, AHe, AHo, AJ, BHe, BHo, BJ, HeHo, HeJ and HoJ, reciprocal crosses pooled) were used in a 51-d experiment (four cows/breed group) to determine apparent and true digestibility of magnesium (Mg). Cows were greater than 10 yr old and averaged 535 kg. Cows were individually fed daily for four consecutive periods at each of four intake levels in a replicated Latin-square design, with replicate being breed group. The diet was primarily cottonseed hulls, cottonseed meal and ground milo with a Mg content of .30% as fed. Intake levels were 50, 83, 117 and 150% of each cow’s estimated maintenance energy requirement. Period 1 consisted of 10 d adjustment and 7 d for each subsequent period followed by a 5 d collection of fecal samples. Fecal samples were collected daily at randomly assigned times. Feed and feces were analyzed for indigestible neutral detergent fiber to determine fecal output and for Mg to determine apparent Mg digestibility. True digestibility of Mg was predicted from the regression of apparent Mg absorption on Mg intake. Apparent digestibility of Mg increased (P<.05) from 13.2 to 43.4% as intake level increased from 50 to 150%. The apparent digestibility of Mg in AHe, J and HoJ was lower (P<.05) than that of B, AB, BHe, BHo, HeJ and BJ. The predicted true digestibility of Mg across all breed groups was 61.1%. Predicted true digestibility of Mg was higher (P<.05) for B, BHo, A and HeHo than He, HeJ, BHe, J and Ho. The estimated endogenous excretion of Mg ranged from 1.8 to 10.5 g/d. Holstein-Jersey and HeHo had a larger (P<.05) quantity of estimated endogenous Mg excreted than AB, He, Ho, HeJ and BHe. Differences observed between breeds of cattle for grass tetany may be partially related to these changes in Mg digestibility.

¹ Journal Paper TA 20864. Texas Agr. Exp. Sta.

² Dept. of Anim. Sci.