Effect of Estrus Activity when Evaluating Feed Efficiency in Heifers

and Implications Estrus activity in virgin heifers tends to affect the outcome in measurements observed during performance and feed utilization tests. Prior to any testing it is prudent to identify those females that have shown estrus activity consistently and those which have not, then evaluate animals for feed efficiency grouped by stage of sexual maturity. Introduction Feed efficiency measurements taken on individual heifers or bulls for the purpose of selection of replacement breeding stock tend to be taken when these animals are 200 to 400 days of age. During this time frame, many of these animals undergo puberty and the subsequent changes advancing sexual maturity initiates; however they do not all mature at the same rate. These changes tend to affect carcass characteristics due to the anabolic nature of the hormones associated with this stage of life, but also there are behavior changes that could have some effect on feed use. Estrus activity of heifers in the feedyard is discouraged and the use of additives such as melengestrol acetate (MGA) which suppresses estrus activity tends to improve feed conversion and weight gain in large pen studies. With this in mind along with the fact that puberty onset is variable within a weaning contemporary group, there was some concern that evaluation of feed efficiency may be