Reproductive Endocrinology of the Female Yak

Summary

In reviewing the literature, this paper assesses the current level of our understanding of the hormonal control of puberty, estrous cycle, pregnancy, parturition and postpartum period in the yak in order to provide basic information for improving reproduction in yaks.

Introduction

Endocrinology and reproductive physiology are rapidly growing areas in the broad field of animal physiology. A good understanding of endocrine mechanisms which control reproduction is important in all animals. The endocrinology of reproductive cycles has been studied in different mammalian species. However, information pertaining to reproductive hormones in yaks remains obscure at present. Since 1985, the author and his co-workers have engaged in the study of reproduction in yaks, with emphasis on reproductive hormones. Research has progressed slowly and much remains to be studied, given that the environment in which yaks live is extremely harsh. The aim of this review, therefore, is to assess our current level of understanding about reproductive endocrinology in an effort to provide basic information for improving yak reproduction.

Before and at the Onset of Puberty

Fifteen yak heifers were used to monitor plasma progesterone concentrations before and at the onset of puberty [1]. Heifers were divided into three age groups: group I (10 - 14 months, n=5), group II (20 - 24 months, n=5) and group III (30 - 36 months, n=5). Yak heifers in Group I were found to have two different progesterone profiles: inactive ovary profile (IO) and low progesterone with short-length cycle profile (LPSC) (Fig. 1). Yaks in group II had three profiles: IO, LPSC and low progesterone with normal-length cycle profile (LPNC) (Fig. 2).

Figure 1. Patterns of plasma progesterone before puberty in Group I yak heifers (n=5). Time= each intervals represents 4 days (see text).

Figure 2. Patterns of plasma progesterone before puberty in Group II yak heifers (n=5). Time= each intervals represents 4 days (see text).

Yaks in group III had three profiles: LPSC, LPNC and a normal estrous cycle profile (Fig. 3). In the present study, it was observed that yak heifers have one, two or more brief rises in circulating progesterone. These increases, however, were not followed by a normal luteal phase except in two yak heifers that showed estrus. This finding is different from literature reports for other cattle.

Figure 3. Patterns of plasma progesterone before and at the onset of puberty in Group III yak heifers (n=5). O1 equals day of estrus (n=14). O2 equals day of estrus (n=15). Time= each intervals represents 4 days (see text).

During the Estrous Cycle at Estrus

Yu and Chen [2] measured concentrations of luteinizing hormone (LH), estradiol-17β and progesterone in peripheral plasma in six yak cows at estrus (Fig. 4).

Figure 4. Mean concentrations (+/- SD) of LH (n=6), oestradiol-17β (n=5) and progesterone (n=5) in peripheral plasma taken at 1 hour intervals around the time of estrus.

The LH peak occurred 12 - 15 hours after the onset of estrus, reaching its maximum level at 14 hours. Estradiol-17β concentrations were high at the onset of estrus, continued to increase and reached a peak value about 2 hours after the LH surge decreasing thereafter. Progesterone concentrations were low at the onset of estrus, rising markedly at 16 hours and peaking 18 hours after the onset of estrus (4 hours after the LH surge). Progesterone concentrations decreased rapidly starting around 19 hours reaching basal levels 20 hours after the onset of estrus.

Overall, the patterns and temporal relationships of LH, estradiol-17β and progesterone at estrus in the yak were similar to those reported for other domestic ruminants, suggesting that mechanisms for the control of ovulation are similar.

Hormone Concentrations during the Normal Estrous Cycle

Concentrations of estradiol-17β and progesterone in plasma were measured in six normally cycling yaks [3] (Fig. 5 and Fig. 6). There were three peaks of estradiol-17β in plasma and milk on the day of estrus, and on days 5 and 14 of the cycle. Progesterone levels in plasma and milk were low during estrus but peaks were observed on day 15. As measured in our laboratory, estradiol-17β and progesterone concentrations in milk were about 4 or 5 times higher than plasma concentrations.

Figure 5. Progesterone concentrations (P4) in plasma and milk just before the breeding season (n=9) and during the normal cycle (n=6) in yaks.

Figure 6. Estradiol-17β concentrations in plasma and milk just before the breeding season (n=9) and during the normal cycle (n=6) in yaks.

Hormone Concentrations During a Short Cycle

The pattern of both oestradiol-17β and progesterone concentrations during the short cycle were similar to those of the normal cycle, however the values were lower [3] (Fig. 7 and Fig. 8).

Figure 7. Progesterone concentrations in plasma and milk during a short estrous cycle in three yaks.

Figure 8. Estradiol-17β concentrations in plasma and milk during a short estrous cycle in three yaks.

Hormone Concentrations During Pregnancy and Periparturient Period

Progesterone profiles were similar for pregnant and non-pregnant yaks 14 days after estrus, however, in pregnant yaks concentrations were significantly higher on Day 19 and tended to increase gradually thereafter (Fig. 9 and Fig. 10).

Figure 9. Progesterone concentrations in plasma and milk during the first month of pregnancy (n=12).

Figure 10. Estradiol-17β concentrations in plasma and milk during the first month of pregnancy (n=12).

Plasma progesterone concentrations decreased rapidly on Day 120, then increased to reach a maximum level on Day 210. Concentrations decreased again 20 days before parturition reaching basal levels at parturition. Estradiol-17β levels in plasma and milk increased gradually from Day 23 after conception, decreased abruptly on Day 60, and increased again to reach a maximum level at parturition. Estradiol concentrations decreased again after parturition to reach similar levels as measured during mating (Fig. 11) [4].

Figure 11. Estradiol-17β concentrations in plasma and during gestation and the periparturient period in yaks (n=8).

Table 1 shows progesterone (P4) and estradiol-17β (E2) concentrations and P4/E2 ratios measured in eight yak cows during the periparturient period [4]. The progesterone to estradiol-17β ratio was very high (265.48) on Day 234 of pregnancy declining abruptly at parturition (Day 0; 0.66). The P4 to E2 ratio increased rapidly again after parturition (reaching 16.7 one day later) maintaining this level for 20 days following parturition. The correlation between plasma progesterone and plasma estradiol-17β within 20 days before parturition was significant (r = -0.8446).

In general, progesterone and estradiol-17β patterns during pregnancy and the periparturient period are similar to those of the dairy cow [5-8]. However, levels of plasma progesterone in yaks declined on Day 90 and increased again on Day 150, similarly to progesterone concentrations in the guinea-pig [9]. This change in progesterone secretion may be due to decreased secretory function of the pregnant corpus luteum by Day 90; the function of the corpus luteum of pregnancy being enhanced by the placenta, as in sheep and guinea-pigs on Day 150. This mechanism needs further study.

The estradiol-17β peak in dairy cows occurs on the day before parturition [10,11] or on the day of parturition [12]. The results of our study in yaks agree with the latter report. The sharp rise of estradiol-17β at parturition may be caused by the pronounced increase in uterine blood flow, as in dairy cows, which may benefit parturition. At parturition, in yaks progesterone levels decline abruptly and the ratio of P4/E2 switches markedly. This is obviously different from the horse [13] and guinea-pig [14], but similar to sheep [15], dairy cows [6,7] and rats [17]. The `progesterone-block' hypothesis may be used to explain the mechanism regulating the onset of parturition in yaks.

Hormone Concentrations During the Postpartum Period

Milk progesterone concentrations in 15 suckled yak cows were determined to monitor postpartum ovarian activity [18]. Yak cows were classified into four groups (Table 2). Type I (normal): 6 cows showed cyclic changes in milk progesterone concentrations within 40 days postpartum. Type II (a short progesterone rise): 2 cows had a short rise in milk progesterone concentration within 20 days postpartum, then progesterone concentrations remained low (<0.5 ng/ml) until 90 days postpartum. Type III (cycle ceased in the presence of low progesterone concentrations): 2 cows had milk progesterone concentrations below 0.5 ng/ml until 90 days postpartum. Type IV (cycle ceased in the presence of high progesterone concentrations): 5 cows had milk progesterone concentrations above 1.0 ng/ml from Day 20 until 90 days postpartum. Among cows classified as type II and III, all four cows had inactive ovaries as determined by rectal palpation. Type IV cows were diagnosed as bearing a persistent corpus luteum as determined by rectal palpation.

Table 3 [19] shows that 66.7% of cows presented luteal activity within 40 days postpartum as determined by progesterone concentrations. However, only 13.3% had shown estrus at the time of estrus observation. Since estrus observations were carried out accurately and by experienced herdsmen and researchers, our results suggest that a high proportion of estrus periods were of low intensity and /or of short duration.

Results indicated that ovarian activity postpartum was re-established earlier in yak cows than in suckled buffaloes, and similarly to dairy cows. Milk progesterone profiles assessed by RIA can be used to monitor postpartum ovarian activity. Progesterone concentrations measured in milk may be helpful in the early detection of ovarian dysfunction in yak cows.