Copper physiology in ruminants: trafficking of systemic copper, adaptations to variation in nutritional supply and thiomolybdate challenge

Ruminants are recognised to suffer from Cu-responsive disorders. Present understanding of Cu transport and metabolism is limited and inconsistent across vets and veterinary professionals. There has been much progress from the studies of the 1980s and early 1990s in cellular Cu transport and liver metabolism which has not been translated into agricultural practice. Cu metabolism operates in regulated pathways of Cu trafficking rather than in pools of Cu lability. Cu in the cell is chaperoned to enzyme production, retention within metallothionein or excretion via the Golgi into the blood. The hepatocyte differs in that Cu-containing caeruloplasmin can be synthesised to provide systemic Cu supply and excess Cu is excreted via bile. The aim of the present review is to improve understanding and highlight the relevant progress in relation to ruminants through the translation of newer findings from medicine and non-ruminant animal models into ruminants.

Ruminants have evolved with the capability to recycle endogenous urea to the gastrointestinal tract (GIT). Ruminal ammonia derived from urea recycling makes a net contribution to digestible N flow if it is used to synthesise microbial protein. The dynamics of urea recycling and its quantitative importance to the N economy of ruminants are affected by dietary and physiological factors. In general, the transfer of endogenous urea to the GIT is related positively to blood urea concentration and rumen-fermentable energy supply and negatively to ruminal ammonia concentration. After consumption of a meal rich in rumen-degradable N, ruminal ammonia concentrations peak and can exceed the rate of carbohydrate fermentation, resulting in inefficient ammonia capture by microbes. These periods are characterised by greater ruminal ammonia efflux and reduced urea influx. A low ruminal ammonia concentration over time can stimulate recycling of endogenous urea-N to the rumen and its capture into microbial protein and reduce N excretion.

High solubility of certain trace minerals (TM) in the rumen can alter nutrient digestibility and fermentation. The objectives of the present studies were to determine the effects of TM source on 1) nutrient digestibility and ruminal fermentation, 2) concentrations of soluble Cu, Zn, and Mn in the rumen following a pulse dose of TM, and 3) Cu, Zn, and Mn binding strength on ruminal digesta using dialysis against a chelating agent in steers fed a diet formulated to meet the requirements of a high producing dairy cow. Twelve Angus steers fitted with ruminal cannulae were adapted to a diet balanced with nutrient concentrations similar to a diet for a high producing lactating dairy cow for 21 d. Steers were then randomly assigned to dietary treatments consisting of 10 mg Cu, 40 mg Mn, and 60 mg Zn/kg DM from either sulfate (STM), hydroxychloride (HTM) or complexed trace minerals (CTM). The experimental design did not include a negative control (no supplemental Cu, Mn, or Zn) because the basal diet did not meet the National Research Council requirement for Cu and Zn. Copper, Mn, and Zn are also generally supplemented to lactating dairy cow diets at concentrations