Читать книгу Bovine Reproduction - Группа авторов - Страница 373

The rearing of replacement heifers is a major financial investment for both beef and dairy cattle producers. The investment expenses do not begin to be recovered until after the first calf is weaned and sold, or in the case of dairy heifers, the onset of lactation, so having heifers calve at an optimal age is paramount to enterprise profitability. For this to occur it is essential that operators know when their heifers have attained puberty and become eligible for breeding. This is most critical for herds using a restricted breeding season.

Observation of signs of estrus can predict onset of puberty but is impractical for application to larger herds. Another observational tool, reproductive tract score (RTS), is a useful predictor of heifer fertility [102]. There is a positive correlation between high RTSs in heifers and percentage of heifers conceiving by artificial insemination [103]. RTS is a subjective estimate of sexual maturity based on ovarian follicular development and diameter of the uterine horns. An RTS of 1 is assigned to heifers with infantile tracts, as indicated by small, toneless uterine horns and small ovaries devoid of significant structures. Heifers with an RTS of 1 are likely the furthest from puberty at the time of examination. Heifers assigned an RTS of 2 are thought to be closer to puberty than those scoring 1, due primarily to larger uterine horns and ovaries. Those heifers assigned an RTS of 3 are thought to be on the verge of estrous cyclicity based on uterine tone and palpable follicles. Heifers assigned a score of 4 are considered to be cycling, as indicated by uterine tone and size, coiling of the uterine horns, and presence of a preovulatory‐sized follicle. Heifers assigned an RTS of 4 do not have an easily distinguished corpus luteum. Heifers with an RTS of 5 are similar to those scoring 4 except for the presence of a palpable corpus luteum [104].