Yields of milk and milk constituents In the National Milk Records (NMR) database are used not only to estimate total lactation yields at the end of a lactation, but also to predict future yields while the lactation is in progress. These predictions are based on the observed yields to date and on the anticipated pattern of future yields, i.e. on a.set of ‘standard’ lactation curves. New curves providing a better fit to the pattern of production of milk, fat, protein and lactose by cattle on a flat rate ration have recently been developed: In ( y ) =a + b ( 1 +t’ /2 ) t’ +ct ’ 2+d/t, where t is the days of lactation, t’ = (t-150 ) / 100 (Morant and Gnanasakthy, 1989). The next objective was to extend the model to take account of the increase In yields frequently observed following the change from winter feeding to grazing.