An empirically based equation for voluntary food intake, obtained from a multibreed cattle experiment, was used to develop formulae for evaluating the overall efficiency of food utilization of beef production in a production unit consisting of a dam and her progeny. With these formulae the dependence of overall efficiency on degree of maturity at slaughter was studied, with reproductive rate, dam food costs, terminal sire size and number of calvings per dam allowed to vary.The formulae were used to study efficiency of food utilization in a traditional beef production system. Maximum efficiency values ranged from 2-3 to 3-5 g of lean tissue per MJ of metabolizable energy. These maximum values were affected to a small extent when progeny were slaughtered much earlier or later than the optimal degree of maturity. In a single-sex, bred-heifer (SSBH) system, the maximum value that could be attained with high technological efficiency and a reproductive rate of unity was 5-2 g, which was 0-5 times more efficient than the highest achievable traditional value. With a reproductive rate of 0-85, the highest efficiency value in an SSBH system was 4-8 g.Overall efficiency of food utilization was also examined in a sex-controlled system where all offspring for slaughter were male and in a modified traditional system where every surplus female was bred once. Both these systems could give higher efficiency (up to 1-08 times) than in a traditional system, but neither could compete with a well-operated SSBH system.Formulae to deal with multiple births, partial sex control and other factors are given in the APPENDIX.

Voluntary food intake and body weight were examined over 4-week intervals between 14 and 70 weeks of age in 306 females from 25 British breeds of cattle. At each age, the relationship of the natural logarithm of voluntary food intake to that of body weight was examined by linear regression both within and between breeds.Of the total variation in voluntary food intake, the proportion accounted for by body weight was extremely high between breeds (phenotypically, 0-80 or more; genetically 0-88 or more, at most ages) but phenotypically low within breeds (0-33 or less). The mean voluntary intake of a breed at any age could be predicted from its mean body weight at the same age with a coefficient of variation (CV) among breeds that declined with age from 0-08 to 0-04. Within breeds, the corresponding CV for individual intake was between 0-12 and 0-15 beyond 9 months of age, and even higher at early ages.Within breeds, the regression coefficient of log intake on log body weight was close to the value of 0-7 at all ages. Between breeds, it was over 0-8 at early ages, declining to about 0-7 beyond 1 year of age. Thus, genetically larger breeds voluntarily consumed relatively more food at early ages compared with later ages. Breed size should therefore be taken into account when recommending food intake requirements. Breed deviations for high and low appetite are discussed.

An attempt is made to meet the need for a single standardized growth equation to cover the entire period from conception to adulthood in a wide range of mammalian species. It is suggested that an ideal equation is one which adequately describes the overall shape of a growth curve and, in addition, can be extended or modified to give greater flexibility and higher precision as necessary. The structure of such an equation is discussed and results in the derivation of a logistic relationship between adjustable ‘flexibility’ functions of size and time.Data from eight species are examined and in both its basic and extended forms the equation is found to give acceptable fits to individual species and to provide a useful mean standardized growth curve for mammals.

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.