title: Labile magnesium reserves in beef cows subjected to different prepasture supplementation regimens.
abstract: Thirty pregnant beef cows were utilized to determine the effects of winter Mg supplementation regimen on blood minerals after cows were turned to a spring tetanigenic tall fescue pasture. The winter Mg treatments were (1) tall fescue hay with free access to a Mg-deficient salt-mineral mix, (2) 6.4 kg of corn silage dry matter plus 114 g MgO/d and (3) tall fescue hay with free access to a salt-mineral mix containing 40% MgO from January 1 to February 15. All cows were then placed on the same tetanigenic pasture from February 15 to April 17 with free access to a Mg-deficient salt-mineral supplement. Forage Mg remained below .2% during the experiment. Forage Ca, P and Al changed throughout the spring, attaining maximum values of .35%, .46% and 415 ppm, respectively. Forage N and K also increased throughout the spring, reaching values of 3.5 and 3.8%, respectively, at the April 3 sampling. The forage K:(Ca + Mg) ratio approached 2.2 by March 26, which coincided closely with the average tetany date (March 29). Serum Mg averaged 1.97, 3.58 and 2.06 mg/dl for treatments 1, 2 and 3, respectively, on February 15 before turning cows to pasture. There were no treatment differences for serum Ca, P and K during the experiment. Eight cows exhibited symptoms of grass tetany (collapse) on an average date of March 29. Winter Mg supplementation provided little long-term protection against hypomagnesemia after turning cows to tetanigenic pasture, indicating that cows must have a supplemental source of Mg during this critical period.
mesh_terms: D000818:Animals; D002136:Calcium, Dietary / Q000008:administration & dosage; D002417:Cattle; D002418:Cattle Diseases / Q000517:prevention & control*; D005260:Female; D005527:Food, Fortified; D008275:Magnesium Deficiency / Q000517:prevention & control / Q000662:veterinary*; D008277:Magnesium Oxide / Q000008:administration & dosage*; D008903:Minerals / Q000097:blood; D010758:Phosphorus / Q000008:administration & dosage; D010939:Plant Poisoning / Q000517:prevention & control / Q000662:veterinary*; D011247:Pregnancy; D011248:Pregnancy Complications / Q000517:prevention & control / Q000662:veterinary*; D012621:Seasons; D013746:Tetany / Q000517:prevention & control / Q000662:veterinary*
chemical_list: D002136:Calcium, Dietary; D008903:Minerals; D010758:Phosphorus; D008277:Magnesium Oxide