Phenotype = Genotype x Environment + Hormones.
An individual animal’s APPEARANCE and PERFORMANCE is determined by its genes and environment which is further MODIFIED by hormones.
When an individual reaches sexual maturity, it’s long bones (frame) slow down in growth and eventually stop growing. OR, as soon as there is an increase in the level of sex hormone (testosterone in males; oestrogen in females) an individual matures sexually and slows down in long bone growth and eventually ceases in frame growth.
The earlier this process (sexual maturity) occurs the smaller the mature frame size will be REGARDLESS of genotype - early and late maturing genotypes will be smaller at maturity. BUT, the decrease in frame size is not directly correlated with lighter weight. Earlier maturing individuals will be fuller packages.
Therefore, physiologically and sexually earlier maturing bulls will have a “pony-type” appearance. Although this is conventionally considered to be undesirable (Bonsma) I believe such animals to be exactly what we require in terms of veld (grass) productivity.
Selection for growth and feedlot feed conversion “efficiency” (size) will favour the bull on left largely due to the fact that he lacks testosterone (growth inhibitor). Selecting for maturity rate (12 month weight / mature size; package fullness) will favour the bull on the right.
Extremes in size and masculinity determined by genotype and testosterone.
Note the relatively wider thighs left) as opposed to wider hips (right). Wide thighs is a masculine trait. Wide hips is a feminine trait.
What applies to the male in terms of hormones, size and sexual maturity also applies to the female. Substitute testosterone with oestrogen.
Same genotype (inherently fat) whose body condition is modified by sex hormones. This underlines the danger of emphasizing detail in selection. Selecting for fat in the male and muscle in the female WILL result in a hormonal imbalance.
