Twenty-two pairs of female Friesian twin calvxs, fed a standard pelleted diet with an ME value of 9·3 MJ 'kg, were split between the following nutritional treatments: (a)ad libitum (b) a weekly amount equal to 24% of body weight or (c) equal to 20% of body weight. Each proportionally-fed animal eventually reached a break-point when the amount of food it was offered exceeded its intake capacity.

For the ‘24%’ and ‘20%’ regimes respectively, break-points occurred at a mean age of 40 and 48 weeks and at a mean body weight of 222 and 258 kg. Surprisingly, the mean body weights at these ages on the ad libitum regime were virtually the same (223 and 265 kg). Thus growth up to break-point was, on average, just as fast for proportional feeding as for ad libitum.

In general, growth up to break-point was significantly slower than the exponential rate expected from a simple linear model. Nevertheless, animals fed at the 20% level grew almost exponentially beyond 6 months of age, and the normally expected decline in the food efficiency ratio was delayed.

The hypothesis being tested was that a proportional feeding system, while in operation, would ensure highly efficient growth and that the break-point could be used as a self-determined optimal slaughter point for each individual. On the evidence obtained, further investigation would be worthwhile, especially at proportional feeding levels giving break-points beyond 1 year of age.