A method for inceasing the weight gain and/or improving the feed utilization efficiency and/or increasing the lean body mass and/or decreasing birth mortality rate and increasing post-natal survival rate of livestock comprises the administration to livestock of an effective, non-toxic amount of a compound of formula (I), or a veterinarily acceptable acid addition salt thereof: ##STR1## wherein R.sup.1 is phenyl (C.sub.1-6)alkyl or optionally substituted C.sub.1-6 alkyl, and W is optionally substituted phenyl, a heterocyclyl group, or phenoxymethyl optionally substituted on the phenyl group. Certain compounds of formula (I) are novel.