Brody (1945) concluded that the inflection point of the growth curve of domestic animals coincides with the onset of puberty and is coupled with an increase in proportion of fat gain at that time. This purported coincidence of growth inflection and onset of puberty has been termed the ‘Brody Law’. Recent findings suggesting a pivotal role of body energy reserves, communicated by the metabolic hormone leptin, on the onset of puberty led to the hypothesis that sheep must reach the inflection point of growth (AIP) considerably before the onset of puberty (AOP). In order to test this hypothesis, growth curves were fitted for ewe lambs on different growth trajectories from two experiments. Both experiments examined the effect of growth trajectory on AOP in ewe lambs. One data set was developed in France with Merino sheep; the other came from two distinct genetic lines of Targhee sheep in the USA. The French experiment subjected ewe lambs to two different feeding levels, while the USA experiment compared two nutritional regimens differing in both energy and protein concentration.
Several non-linear models described in the literature as potentially useful for modelling weight–age relationships were fitted. The Logistic function was identified as the superior model for all datasets. All animals in both experiments reached AIP considerably before AOP. AOP was defined using two criteria threshold levels of progesterone as AOP1 ⩾0·5 ng/ml and AOP2 ⩾1 ng/ml progesterone. For the USA data, multivariate analysis of AIP, AOP1 and AOP2 demonstrated that nutritional treatment was highly significant; this was also the case for the multivariate analysis of AIP, degree of maturity at AOP1 (DOM1) and DOM2. The correlation between AIP and DOM was highly negative. In contrast, the feeding treatment in the French experiments had no effect on any of the response variables except estimated mature weight. However, AIP was negatively correlated with DOM1 and DOM2. AOP was highly positively correlated with DOM. Most notably, AIP was not correlated with AOP1 or AOP2 in either experiment, and all animals reached AIP considerably before AOP1. These findings are consistent with the hypothesis that growing female sheep must reach a certain minimum level of body fatness before the onset of puberty. Genetic group and nutritional treatment significantly affect growth curve parameters; hence, the age at which this (currently unknown) level is reached must strongly depend on nutritional regimen. It is concluded that growth modelling per se cannot be used to infer onset of puberty, and that endocrine thresholds do not necessarily demarcate distinct phases of the growth curve.