268. Copper Deficiency of Cool-Season Invaded Grassland Affects Liver Mineral Content of Growing Beef Heifers.

Abstract Yearling beef heifers grazing grasslands often exhibit outward signs of copper (Cu) deficiency, including poor coat color, dulled hair and weakened immune response, indicating inefficiencies in growth and production. We utilized a 2 by 2 factorial study to compare liver mineral concentration and weight gain of yearling heifers (Bos taurus) receiving one of two types of Cu supplement (amino acid complexed copper vs. copper chloride) and grazing on cool-season dominated pastures that were managed two different ways (rested one or more years prior to grazing vs grazed yearly). Three different pastures were split in 4 paddock strips, with two assigned for organic supplementation and two for inorganic supplementation of copper with a group of 3 heifers (blocked by weight) grazing in each sub paddock. Pastures contained primarily smooth bromegrass (Bromus inermis Leyss.), Kentucky bluegrass (Poa pratensis L.), western wheatgrass [ Pascopyrum smithii (Rydb.) A. Liive], needle and thread (Stipa comata Trin. and Rupr.), green needlegrass (S. viridula Trin.), blue grama [Bouteloua gracilis(Willd.ex Kunth) Lag. Ex Griffiths] and upland sedges (Carex spp.). Liver samples were collected before and after grazing the trial pastures for 6 wk. for two years, while due to drought conditions, a smaller group of cattle were grazed the other two years. Therefore, four years of weekly forage samples and biomass data were collected but only two years of biopsied liver, forage and mineral consumption, and weight data. Heifers were supplemented with loose mineral supplying 168 mg Cu hd-1d-1. Liver Cu concentration in the pre-trial sampling averaged 29 ±21.01 mg/g DW for both groups. Post trial liver Cu was increased 35 mg/g DW for heifers receiving amino-acid complexed copper, and 29.8 mg/g DW for those on inorganic sources, but the difference was not significant and was not affected by pasture management (P = 0.56). Forage samples did not demonstrate differences in Cu concentration due to management. There were also no differences in body weight increase between amino acid Cu- supplemented and Cu-chloride supplemented heifers during the grazing season. Average daily gain was significantly different due to year (0.65 kg the first year and 0.46 kg, P = 0.02 the next year) but not affected by copper source or pasture management. The most consistent benefit from Cu supplementation was an increase in liver copper concentration regardless of copper source. Forage values indicate that grassland forages are marginal to deficient in Cu for optimal development of beef heifers and supplementation is necessary, with amino-acid chelated Cu heifers posing a marginally greater Cu content in liver.