A muscle biopsy technique was employed in an experiment designed to assess the effects of hexoestrol implantation on 2-year-old and 6-year-old Aberdeen Angus steers under New Zealand grazing conditions. This paper reports the results concerning live-weight gains, carcass weights and detailed analyses of samples of m. semitendinosus and associated subcutaneous fat.There was some evidence of an adverse effect of the biopsy operation on the older, less tractable, animals, despite the observation of no apparent effect of repeated muscle biopsies in a preliminary trial with sheep.Hexoestrol implantation, of four 15 mg. pellets for the 2-year-old steers and six 15 mg. pellets for the 6-year-olds, led to significant increases in live weight gains during the later part of the trial only, and reduced the dressing-out percentage of the younger animals. Although there were indications of a positive response in carcass weight to hexoestrol implantation, it was concluded that, under the conditions of this experiment, implantation of relatively large dosages was not economically justified in terms of increased meat production.Absorption of the implanted pellets varied from 42 to 85% of the total dosage, but evidenced little association with live weight changes.Deposition of subcutaneous fat appeared to be reduced under the hormone treatment, but the responses in this respect of the 6-year-old steers were highly variable.

Field trials on hexoestrol implantation were conducted in the Waikato area of the North Island of New Zealand during 1958–59, involving 260 two and three year old Aberdeen Angus steers from nine farms. Two levels of hormone implantation were studied, namely, 30 and 45 mg. The trials ranged in duration from three to five months with an average of four months.Pronounced differences were apparent between farms in the overall growth rates and carcass quality scores of the cattle.Hexoestrol implantation led to a significant increase in carcass weight, estimated at approximately 22 lb., with little evidence of real differences between farms in the average response to the hormone.Response to the higher as compared with the lower dose of hexoestrol varied markedly among the separate farms. In general, the lower dose of 30 mg. tended to yield the greater increase in carcass weight.A small, but consistent, depression of carcass quality scores was manifest under hormone treatment, with no real differences between the two dosage levels.