What is defined as the gradual increase in size and weight of an animal as it ages?

The concept being described in the question is best captured by the term "growth." Growth refers specifically to the gradual increase in size and weight of an animal as it ages, which encompasses various physiological changes that occur during an animal's life cycle. This process is critical in the context of agriculture, as understanding the growth stages of livestock can impact management practices, nutrition, and overall productivity.

Development, while related, encompasses broader changes beyond just size and weight, including changes in function and maturation. Maintenance involves the upkeep of bodily functions and does not specifically address change in size or weight. Reproduction pertains to the biological processes that lead to the production of offspring, which, while important, does not directly describe the gradual increase in size and weight of an individual animal. Therefore, "growth" distinctly identifies the phenomenon of increasing size and weight over time.