Introduction
During the last decade, the US swine industry has changed production methods to prevent transmission of diseases from the sow to the litter. Through segregated early weaning (SEW), the industry is now producing healthier pigs that grow at faster rates with greater efficiencies. Although these changes have improved health and performance of pigs, they have resulted in more variable and less predictable reproductive performance of weaned sows (2, 4, 5, 6). Weaning litters early, especially at less than 2 weeks of age may lead to several problems:
Poor reproductive performance associated with early weaning may increase nonproductive sow days and reduce economic returns. Furthermore, unpredictable reproductive performance hinders use of all in all out management of the farrowing house and reduces throughput in breeding, gestation and farrowing facilities.
Reproduction During Early Lactation and After Early Weaning
In order to understand why early weaning may lead to breeding problems, it is necessary to understand the normal process that occur during the transition from pregnancy to lactation to the point where the sow is capable of breeding back in a normal manner. Studies in our laboratory at N. C. State have provided a clear understanding of what happens during this period and why some sows fail to reproduce normally after early weaning. This paper deals with the results of these studies (1, 7, 8, 9, 10, 11).
After delivering her litter, a sow's reproductive system has to undergo several changes in order for her to breed back for another litter. Her reproductive system must:
Follicle growth. We discovered that on the day of farrowing, sows already have ovulatorysized (> 6 mm diameter) follicles on their ovaries (8, 10). This was a surprising finding, because other farm animals like cattle and sheep have no follicular activity at the time of delivery. In our studies, we found that the follicles on a sow's ovary disappeared in about 48 hours if she nursed a litter, because nursing inhibited secretion of the gonadotropic hormones, follicle stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary (10). After about 2 to 3 weeks of nursing, the ovulatory sized follicles began to reappear (8, 10), because the nursing activity had begun to decline and the suppression of FSH and LH was not as great (Figure 1).
We also found, as described subsequently in this paper, that many sows were unable to ovulate follicles during the first 2 weeks after farrowing, so if the litter is weaned at birth (zero weaning) or shortly thereafter, these ovulatory sized follicles continue to grow and form large cystic follicles. Large cystic follicles may cause the sow to show continuous signs of heat or they may cause her to be anestrus for several weeks. In either case, the cystic follicles cause infertility.
Expression of heat. To determine whether sows are able to show a normal heat at 14 and 28 days after farrowing, we gave sows a single injection of estrogen (estradiol benzoate) at a dose rate (10 g/kg) known to cause a normal heat (9). Some of the sows were nursing litters (lactating, L), but some had their litters weaned at the time of estrogen injection (weaned at 0 hours, W0) and some had their litters weaned 96 hours before estrogen treatment (weaned at 96 hours, W96). These sows were checked for heat with a boar. Of 32 sows given a dose of estrogen, 31 showed a normal heat (Figure 2). There was no difference between sows treated on day 14 or 28 of after farrrowing or between those nursing litters and those that had their litters weaned 0 or 96 hours before injection of estrogen. Thus, sows were capable of showing a normal heat at 2 and 4 weeks after farrowing.
Elicit a normal surge of LH. In order for sows to ovulate mature follicles, their hypothalamus and pituitary must respond to estrogen by eliciting a surge of LH. This occurs because the sow's hypothalamus produces a surge of gonadotropin releasing hormone (GnRH) in response to estrogen (3). This GnRH from the hypothalamus stimulates release of LH from the sow's pituitary. The LH from the pituitary stimulates the mature follicles to ovulate.
The number of sows that showed a surge of LH in response to estrogen is illustrated in (Figure 3). Among 16 sows treated on day 14 after farrowing, 11 failed to elicit a surge of LH, even though 10 of the 11 showed heat (Figure 2). In contrast, by day 28, 15 of 16 sows elicited an LH surge, and all of them showed heat (Figure 2).
These results show that even if sows come into heat within 10 to 14 days after zero weaning or early weaning, many will not ovulate because their reproductive system is not capable of eliciting a normal surge of LH. The problem is not associated with a deficiency of LH, because the amount of LH stored in the sow's pituitary actually increased from day 1 to 14 (7, 8, 10). The problem is that the normal feedback mechanism from the ovary (estrogen) to the hypothalamus (GnRH) to the pituitary (LH) does not function properly during the first two weeks after farrowing.
Ovulate in response to estrogen and LH surge. After treatment of sows with estrogen, we measured concentrations of progesterone in blood samples collected 2 to 3 times weekly from sows to determine if they had ovulated. Progesterone is secreted by the corpora lutea, which are formed on the ovary after follicles ovulate. Progesterone is the ovarian hormone that is necessary for establishment and maintenance of pregnancy.
Among the 5 sows that had an LH surge after treatment with estrogen on day 14, 3 ovulated in response to the LH surge (Figure 4), and all 3 of these had their litters weaned for 96 hours before estrogentreatment (compare Figures 3 and 4). Thus on day 14, only 60% of the sows that experienced a surge of LH actually ovulated. By day 28 the response had improved for weaned sows because 8 of 8 ovulated in response to the LH surge (compare LH surges in Figure 3 with ovulations in Figure 4). However, sows that were still nursing litters on day 28 did not ovulate, even though 7 of 8 had shown a surge of LH.
These results suggest that nursing may interfere with the ability of follicles to ovulate during the first 4 weeks of lactation, even if sows have a surge of LH. Exactly how nursing may block ovulation is unknown. However, if sows show heat and have an LH surge soon after pigs are weaned during early lactation, then the ovulatory response appears to be normal.
Normal uterine function. The sow's uterus must involute after farrowing and return to a nonpregnant size quickly in order for fertility to be restored. Uterine involution is necessary in order for the reproductive tract to transport spermatozoa after mating and embryos after fertilization. In our research, we have not measured uterine involution, but studies by others have shown that the sow requires about 2 weeks after farrowing for her uterus to return to a state where conception is near normal (4, 12). Allowing at least 2 weeks from farrowing to mating allows the uterus to involute substantially, but a 3 week period is necessary for involution to be complete.
Take-Home Message
Suggested Reading
1. Armstrong, J. D., N. M. Cox and J. H. Britt. 1986. Seasonal differences in function of the hypothalamichypophysialovarian axis in weaned primiparous sows. J. Reprod. Fertil. 78:11.
2. Britt, J. H., J. D. Armstrong, N. M. Cox and K. L. Esbenshade. 1985. Control of follicular development during and after lactation in sows. pp. 3754 in Control of Pig Reproduction II, G. R. Foxcroft, D. S. A. Cole & B. J. Weir (eds.). Journals of Reproduction and Fertility, Ltd., Cambridge.
3. Britt, J. H., K. L. Esbenshade and A. J. Ziecik. 1991. Roles of estradiol and GnRH in controlling negative and positive feedback associated with the LH surge in ovariectomized pigs. Biol. Reprod. 45: 478.
4. Cole, D. J. A., M. A. Varley and P. E. Hughes. 1975. Studies on sow reproduction. 2. The effect of lactation length on subsequent reproductive performance of the sow. Anim Prod. 20: 401.
5. Dial, G. D. and J. H. Britt. 1986. The clinical endocrinology of reproduction in the pig. pp 905911 in Current Therapy in Theriogenology 2, D. A. Morrow (ed), W. B. Saunders, Philadelphia.
6. Esbenshade, K. L, A. J. Ziecik and J. H. Britt. 1990. Regulation and action of gonadotropins. J. Reprod. Fertil. Suppl. 40: 19.
7. Sesti, L. A. C. and J. H. Britt. 1993. Agonistinduced release of gonadotropinreleasing hormone, luteinizing hormone and folliclestimulating hormone and their associations with basal secretion of luteinizing hormone and folliclestimulating hormone throughout lactation in sows. Biol. Reprod. 49: 332.
8. Sesti, L. A. C. and J. H. Britt. 1993. Relationship of secretion of GnRH in vitro to changes in pituitary concentrations of LH and FSH and serum concentrations of LH during lactation in sows. J. Reprod. Fertil. 98: 393.
9. Sesti, L. A. C. and J. H. Britt. 1993. Influence of stage of lactation, exogenous luteinizing hormonereleasing hormone, and suckling on estrus, positive feedback of luteinizing hormone, and ovulation in sows treated with estrogen. J. Anim. Sci. 71: 989.
10. Sesti, L. A. C. and J. H. Britt. 1994. Secretion of gonadotropins and estimated releasable pools of gonadotropinreleasing hormone and gonadotropins during establishment of sucklinginduced inhibition of gonadotropin secretion in the sow. Biol. Reprod. 50: 1078.
11. Stevenson, J. S., N. M. Cox and J. H. Britt. 1981. Role of the ovary in controlling LH, FSH and prolactin secretion during and after lactation in pigs. Biol. Reprod. 24:341.
12. Varley, M. A. and D. J. A. Cole. 1976. Studies in sow reproduction. 5. The effect of lactation length of the sow on the subsequent embryonic development. Anim. Prod. 22: 79.
