Yields of milk and its constituents recorded by National Milk Records (NMR) are used to estimate total lactation yields and to predict future yields while the lactation is still in progress. Morant and Gnanasakthy (1989) developed new lactation curves of the form:

where t is the days since calving and t' = (t-150 ) /100, for cows on a complete diet throughout lactation. The objective of this study was to extend this model for use with cows at grass during the summer months.

Approximately 2000 2nd and 3rd lactations of Friesian/Holstein cows, 1013 from the North and 960 from the South West of England, were obtained from the NMR data base by systematic sampling. The sampling fraction was 0.1. The cows calved between October 1987 and September 1988. Their lactations spanned at least 305 days and were recorded at approximated monthly intervals. In each of two regions the average milk yield in each month of lactation for cows calving in each month of the year were calculated. To obtain Initial estimates of grazing effects on the shape of the lactation curves, twelve dummy variables (gi, i=1, 2…12), where gi=1 for ith calendar month of recording and 0 otherwise, were included in the model.