Follicular wave of the ovulatory follicle and not cyclic status influences fertility of dairy cows

The objective of this study was to assess objectively the ability of observers to assess body condition of dairy cows. Four observers independently assigned a body condition score (five-point scale, .25 increments) and described the appearance of seven body regions of 225 Holstein cows. Areas described were the thurl region, ischial and ileal tuberosities, ilio-sacral and ischio-coccygeal ligaments, transverse processes of the lumbar vertebrae, and spinous processes of the lumbar vertebrae. An absolute body condition score was designated for each cow based on the modal body condition score for all observers. If there was no modal body condition score, the mean score was used for the absolute body condition score. Statistical analysis of principal components was used to examine the relationship between body region description and absolute body condition score. Descriptions of body regions were highly correlated across all absolute body condition scores. Four principal component vectors explained 83.6% of the variation of the body region correlation matrix. The first principal latent vector accounted for 55% of the variation and was uniformly correlated with all body regions. Analysis of variance of first principal latent vector as the dependent variable and absolute body condition score as the class variable indicated that body condition could be separated into .25 units between 2.5 and 4.0, inclusively. Below 2.5 and > 4.0, body condition could only be separated by .5 units. Distinct changes in specific body regions were associated with change in absolute body condition score. Observers agreed with the absolute score 58.1% of the time, deviating by .25 units 32.6% of the time. A body condition score can be given to a cow based on principal descriptors of specific body regions between 2.5 and 4.0 by .25 units.

Reproductive failure in inseminated cattle results from poor fertilization and embryo survival. Recent studies utilizing dairy and beef cattle indicate that fertilization rates are higher for nulliparous dairy and beef heifers and nonlactating beef cows than lactating beef and dairy cows and nonlactating dairy cows. Several factors affect fertilization rates, but the greatest impact was observed for high producing cows under heat stress, when fertilization was only 55%. Once fertilization has occurred, the fate of a successful pregnancy is then determined by the survival of the embryo and fetus. Losses of pregnancy are characterized by early embryonic death, which occurs prior to the period of corpus luteum (CL) maintenance in the cow at days 15-17 of the cycle, and late embryonic death, which occurs from CL maintenance to the end of the differentiation stage, at approximately 42 days of gestation. After 50 days of gestation, pregnancy losses are less frequent and characterize fetal death. Most pregnancy losses occur prior to the period of maintenance of the CL, but in high producing lactating dairy cattle, substantial losses continue to occur up to 42-56 days after insemination. Several factors affect pregnancy losses in cattle, such as compromised oocytes, which result in poorly developed embryos incapable of cross-talking with the endometrial epithelial cells, to inadequate uterine environment and infectious agents resulting in death of the embryo from undernourishment. Recently, studies have indicated that anovulation/anestrous, the metabolic status of the animal, some dietary ingredients, as well as occurrence of diseases, predispose the cow to experience embryonic and fetal death. Although some insemination protocols might impact embryo survival, when timed AI has been implemented properly, it has not influenced embryonic or fetal death in cattle. Improvements in reproductive programs in the future will have to focus on enhancing fertilization rates and minimizing embryonic losses to optimize conception rates in dairy and beef cattle.

In many nonprimate mammalian species, cyclical regression of the corpus luteum (luteolysis) is caused by the episodic pulsatile secretion of uterine PGF2alpha, which acts either locally on the corpus luteum by a countercurrent mechanism or, in some species, via the systemic circulation. Hysterectomy in these nonprimate species causes maintenance of the corpora lutea, whereas in primates, removal of the uterus does not influence the cyclical regression of the corpus luteum. In several nonprimate species, the episodic pattern of uterine PGF2alpha secretion appears to be controlled indirectly by the ovarian steroid hormones estradiol-17beta and progesterone. It is proposed that, toward the end of the luteal phase, loss of progesterone action occurs both centrally in the hypothalamus and in the uterus due to the catalytic reduction (downregulation) of progesterone receptors by progesterone. Loss of progesterone action may permit the return of estrogen action, both centrally in the hypothalamus and peripherally in the uterus. Return of central estrogen action appears to cause the hypothalamic oxytocin pulse generator to alter its frequency and produce a series of intermittent episodes of oxytocin secretion. In the uterus, returning estrogen action concomitantly upregulates endometrial oxytocin receptors. The interaction of neurohypophysial oxytocin with oxytocin receptors in the endometrium evokes the secretion of luteolytic pulses of uterine PGF2alpha. Thus the uterus can be regarded as a transducer that converts intermittent neural signals from the hypothalamus, in the form of episodic oxytocin secretion, into luteolytic pulses of uterine PGF2alpha. In ruminants, portions of a finite store of luteal oxytocin are released synchronously by uterine PGF2alpha pulses. Luteal oxytocin in ruminants may thus serve to amplify neural oxytocin signals that are transduced by the uterus into pulses of PGF2alpha. Whether such amplification of episodic PGF2alpha pulses by luteal oxytocin is a necessary requirement for luteolysis in ruminants remains to be determined. Recently, oxytocin has been reported to be produced by the endometrium and myometrium of the sow, mare, and rat. It is possible that uterine production of oxytocin may act as a supplemental source of oxytocin during luteolysis in these species. In primates, oxytocin and its receptor and PGF2alpha and its receptor have been identified in the corpus luteum and/or ovary. Therefore, it is possible that oxytocin signals of ovarian and/or neural origin may be transduced locally at the ovarian level, thus explaining why luteolysis and ovarian cyclicity can proceed in the absence of the uterus in primates. However, it remains to be established whether the intraovarian process of luteolysis is mediated by arachidonic acid and/or its metabolite PGF2alpha and whether the central oxytocin pulse generator identified in nonprimate species plays a mediatory role during luteolysis in primates. Regardless of the mechanism, intraovarian luteolysis in primates (progesterone withdrawal) appears to be the primary stimulus for the subsequent production of endometrial prostaglandins associated with menstruation. In contrast, luteolysis in nonprimate species appears to depend on the prior production of endometrial prostaglandins. In primates, uterine prostaglandin production may reflect a vestigial mechanism that has been retained during evolution from an earlier dependence on uterine prostaglandin production for luteolysis.