Selenium is an essential nutrient for dairy cattle, being a key component of the anti-oxidative protection mechanism. With over 20 seleno-proteins being identified in the body as important nutrients or co-factors for the physiological processes involving production, health and fertility, the need for ensuring a selenium supply which is both efficacious and safe has increased dramatically. Key roles of selenium include supporting the immune function and fertility. In terms of reproduction its main action is to protect the developing embryo from oxidative damage. Concern over the sufficiency of selenium supply has increased in recent years and it is well recognised that the British Isles is deficient in selenium as measured in soils and crops. As a consequence, the animal is totally dependent for its selenium supply on concentrated feedstuffs and mineral supplements. There is a cautious approach to dietary supplementation of selenium because of its toxicity even though a legal limit of 0.50 mg/kg in dairy complete feeds applies. It is also recognised that traditional sources of selenium (sodium selenite) have limited availability and utilisation within the animal. Considerable research effort has been directed at evaluating the key selenium metabolite “L-Selenomethionine” as a potential dietary selenium source. Much of the naturally supplied selenium from feedingstuffs is in this form. Supplying a significant proportion of selenium in the organic methionine form has been found to both reduce potential toxicity problems and boost absorption. The aim of this trial was to compare two selenium supplements, selenium selenite (inorganic) and selenium metalosate (organic) in the dairy cows diet and investigate effects on milk yield, milk composition and hygiene, and fertility.