The selection of replacements using applications such as reproductive tract scores and antral follicle count may be useful to eliminate less fertile animals before breeding. Previously, we reported that heifers with increased numbers of follicles achieved puberty earlier than heifers with diminished numbers of follicles. We, therefore, hypothesized that heifers that achieved a reproductive tract score ≥4 earlier in the breeding season would have greater numbers of follicles than heifers that achieved a reproductive tract score ≥4 later in the breeding season. The uterus and ovaries of three hundred and fifty beef heifers from a commercial farm were evaluated by transrectal ultrasonography 20 days before breeding and parameters of growth and development were recorded. The antral follicle count (AFC) of both ovaries was recorded and the heifers assigned a reproductive tract score (1 to 5). Only heifers with a reproductive tract score ≥4 entered to two fixed timed artificial insemination (FTAI) programs. The first group (FTAI¹_, n = 269) started the day of the beginning of the breeding period and the second group (FTAI², n = 61) one month later. The rest were naturally mated (no FTAI, n = 20). Serial pregnancy scanning was performed throughout the breeding season and 45 days after bull removal to estimate the day of conception. Data indicative of growth and development were analysed using the REG and GLM procedures of SAS with FTAI (0, 1 and 2), AFC (low, medium and high), and the interaction as the independent variables. Reproductive tract development was negatively associated to AFC (P>0.0001). There was no interaction between FTAI and AFC (P>0.11). Heifers inseminated at FTAI¹ were heavier and taller, had greater deposition of fat and muscle, and conceived earlier than those inseminated at FTAI² (P<0.05); however, there was no difference in AFC between heifers inseminated in FTAI¹ and FTAI² (15.6 ± 0.4 vs. 16.7 ± 0.9 follicles, respectively; P = 0.3). Furthermore, conception rates did not differ statistically between heifers in FTAI¹ and FTAI² (P = 0.12). Antral follicle count influenced growth and carcass traits (P>0.05) and heifers in the High AFC class conceived approximately 1 week earlier than heifers in the Low and Medium AFC class (P<0.05). We conclude that pubertal heifers have less AFC, thus similar numbers were observed for heifers in both FTAI programs and no differences in pregnancy rate. Antral follicle count may allow for the selection of heifers that conceive earlier, but may result in a lower pregnancy rate.

使用诸如生殖道评分和肛门卵泡计数之类的应用选择替代品可能有助于消除繁殖前繁殖力较差的动物。以前，我们报道说，卵泡数量增加的小母牛比卵泡数量减少的小母牛更早进入青春期。因此，我们假设在繁殖季节较早达到生殖分数≥4的母牛比在繁殖季节较晚达到生殖分数≥4的母牛具有更多的卵泡。在繁殖前20天，经直肠超声检查了来自商业农场的350头小母牛的子宫和卵巢，并记录了它们的生长和发育参数。记录两个卵巢的窦卵泡计数（AFC），并给小母牛分配生殖道分数（1至5）。只有生殖道分数≥4的小母牛才参加两个固定时间的人工授精（FTAI）计划。第一组（FTAI^{如图1所示}_， n  = 269）开始繁殖期的第二天，第二组（FTAI ²，n  = 61）一个月后开始。其余的人自然交配（无FTAI，n  = 20）。在整个繁殖季节和除去公牛后45天进行连续妊娠扫描，以估计受孕的日期。使用SAS的REG和GLM程序（FTAI（0、1和2），AFC（低，中和高））以及相互作用作为自变量，分析了指示生长和发育的数据。生殖道发育与AFC呈负相关（P > 0.0001）。FTAI和AFC之间没有相互作用（P > 0.11）。FTAI ¹授精的小母牛较FTAI ²受精者更重，更高，脂肪和肌肉沉积更早（P <0.05）；然而，在FTAI ¹和FTAI ^2中受精的小母牛之间的AFC没有差异（分别为15.6±0.4和16.7±0.9卵泡；P  = 0.3）。此外，在FTAI ¹和FTAI ^2中的小母牛之间受胎率没有统计学差异（P  = 0.12）。窦房卵计数影响​​生长和FC体性状（P > 0.05），而高AFC级小母牛受孕的时间比中，低AFC级小母牛早1周（P<0.05）。我们得出的结论是，青春期小母牛的AFC较少，因此在两个FTAI计划中，小母牛的数量均相似，并且妊娠率没有差异。窦卵泡计数可能允许选择较早受孕的小母牛，但可能导致较低的怀孕率。