Horn growth rate does not appear to be related to the amino acid profile of the major protein source for dairy cattle (Offer, Logue & Roberts, 1997), but it is possible that sulphur amino acids are limiting in early lactation, when the homeorhetic drive to milk production is most extreme. Supplementation of a high production ration for dairy cows with protected methionine should increase milk production, and allow any sulphur amino acid limitation on horn growth to be alleviated (Mengal, Galbraith, Souri & Scaife, 1997).

A total of 60 in-calf Holstein heifers were divided into two groups in a randomised block design. The study commenced approximately three weeks pre-calving until 26 weeks post calving, and animals were housed in one of three systems for the duration. Two diets were formulated based on a grass silage:maize silage mixture (50:50 DM basis), with rolled wheat, soya bean meal, sugar beet feed and rapeseed meal and were offered from approximately five days post calving for the remainder of the 26 week period.