In selective breeding, what is primarily evaluated in animals to determine breeding worth?

In selective breeding, performance records are primarily evaluated to determine the breeding worth of animals. This approach focuses on quantifiable traits and historical data that demonstrate how well an animal has performed in comparison to others within its breed, as well as its ability to transmit favorable traits to its offspring. Performance records typically include information related to growth rates, milk production, feed efficiency, and reproductive success, which provide valuable insights into the animal’s genetic potential.

While physical appearance, health status, and age can play roles in assessing an animal, they are not the primary indicators of breeding value. Physical appearance can sometimes be misleading as it may not directly correlate with the animal's genetic capabilities. Health status is crucial for ensuring the wellbeing of the animal and its ability to perform, but it doesn't necessarily indicate genetic worth. Age can affect reproductive capabilities or performance but is not an inherent measure of an animal's genetic quality or potential for producing desirable traits in progeny. Therefore, the use of performance records stands out as the most objective and informative basis for making selective breeding decisions.