Access to pasture or good quality grass silage should guarantee an adequate supply of ß-Carotene to dairy cows. However, modern rations for high yielding dairy cows are often based on a total mixed ration (TMR). TMRs in early lactation frequently contain more than 50% maize silage and cows usually have little access to pasture. The capacity of grass products to compensate for the low ß-Carotene content in maize based rations is often overestimated. An inadequate supply of ß-Carotene leads to a dramatic decline in reproductive performance.