Feeding altrenogest during late lactation improves fertility of primiparous sows nursing smaller litters

Abstract

The objective of this study was to determine whether feeding altrenogest (AT) to primiparous sows with smaller litters during the last week of lactation would improve their fertility. At day 21 of a 28-day lactation, 40 primiparous sows nursing ≤ 8 pigs were assigned equally to 2 groups, either to be fed 20 mg per day of AT for the last 7 days of lactation or to serve as untreated controls. To detect estrus, sows had daily fence-line contact with a mature boar. At estrus detection, sows were subject to an ovarian examination with transrectal ultrasound and preovulatory follicles (≥ 0.6 mm) were counted. Sows were artificially inseminated at estrus detection and again 24 hours later. Compared to controls, the AT-fed sows had longer weaning-to-estrus intervals (WEIs; P < 0.001), more pre-ovulatory follicles (P < 0.001), and larger subsequent litter sizes (P = 0.03). Farrowing rates were unaffected by treatment. These data suggest that sows nursing small litters are more likely to initiate a follicular phase during lactation, but that feeding AT prevents this and increases ovulation rate and subsequent litter size.

A primary constraint to consistent weaner output is the availability of enough service-ready gilts and sows when required for breeding (1), which is influenced by weaning-to-estrus intervals (WEIs). It is established that long WEIs (≥ 6 d) result in shorter estrus durations, lower farrowing rates, and smaller litter sizes (2,3), probably due to poor timing of insemination relative to ovulation. Very short WEIs (≤ 3 d) are similarly associated with poorer fertility (2) and, while not specifically investigated, this is also presumably due to poor timing of insemination. Causes of variation in WEIs have previously been reviewed and it was noted that WEI was impacted by differences in the size of the ovarian follicles at weaning (4). Smaller follicles at weaning, which are presumably due to variations in sow nutrient intake caused by the temperature of their environment during lactation, were associated with longer WEIs. This would account for the usually longer WEI of primiparous sows.

Ovarian follicles grow and regress in waves during late lactation and WEI and oocyte quality can be influenced by follicle size at weaning and when sows are bred relative to the stage of this wave (4). Feeding altrenogest (AT) to primiparous sows from the day before weaning suppresses the release of luteinizing hormone (LH), which limits follicle growth to 4 to 6 mm and blocks estrus, thus allowing a longer period of metabolic recovery after lactation. The LH pulsatility suppression lasts for about 5 h after 2 daily AT feeds (5) and for more than 12 h after feeding AT for 13 d (6). When pulsatile release of LH resumes, the follicular phase ensues and sows return to estrus in about 6 d. In addition to metabolic recovery, the effect of AT likely involves synchronization of follicle waves with mating.

Follicular growth may start earlier in sows with small litters (≤ 8 piglets) as they are subject to lower metabolic demands during lactation. These sows may experience late lactation estrus or a very short WEI, which impairs the predictability of the WEI and potentially reduces their fertility after breeding. Feeding AT during late lactation to sows with normal litter sizes has been associated with a tendency to increase subsequent litter sizes (7,8). Therefore, we hypothesized that feeding AT to primiparous sows nursing small litters during the last week of lactation will prevent resumption of follicle growth during lactation, resulting in normal and synchronized WEI and increased subsequent litter sizes.

This pilot study was conducted on a 1200-sow commercial farrow-to-finish farm in Nakhon Pathom, Thailand from August 2014 to May 2015. It was approved by the Institutional Animal Care and Use Committee at Chulalongkorn University. At day 21 of a 28-day lactation, 40 primiparous Landrace-Yorkshire sows were selected that were nursing 8 or fewer piglets. Sows were divided equally into 2 groups to either receive orally 20 mg/d of altrenogest (AT) (Virbagest; Virbac Animal Health, Nonthaburi, Thailand) during the last 7 d of lactation, including the day of weaning (n = 20), or to serve as non-treated controls (n = 20).

To facilitate estrus detection, sows had daily fence-line contact with a mature boar from the day after weaning. At estrus detection, all sows were subject to an ovarian examination with transrectal ultrasound using a 5-MHz convex probe (HS-2000; Honda Electronics, Tokyo, Japan) and the preovulatory follicles (≥ 0.6 mm) were counted. Sows were inseminated at estrus detection and again 24 h later with 3 × 10⁹ sperm in 80-mL of extender. From weaning, sows were housed individually in gestation stalls in an open-sided barn and were fed 3 kg/d of a commercially formulated gestation diet. During gestation, feed allowance was 2 kg/d. Water was freely available from nipple drinkers. Data recorded were WEI, number of preovulatory follicles immediately before insemination, and subsequent farrowing rates and litter sizes. All analyses were conducted with SAS 9.1 (SAS Institute, Cary, North Carolina, USA) and data are presented as mean ± SD. The WEI, number of preovulatory follicles, and litter sizes were tested for normality using the UNIVARIATE procedure option NORMAL. As the number of preovulatory follicles did not follow a normal distribution, they were compared using Mann-Whitney U-test. Weaning-to-estrus intervals (WEIs) and total-born piglets were compared using t-test and farrowing rates were compared using Fisher’s exact test.

All sows exhibited estrus and were inseminated. The control group had a shorter WEI than the AT-fed group (2.7 ± 1.0 versus 5.7 ± 1.4 d, respectively; P ≤ 0.0001). The farrowing rates were not significantly different between treated and control groups (95% versus 90%, respectively), which reflected the excellent breeding management on this farm. Compared to the control sows, however, the AT-fed sows had more preovulatory follicles (22.7 ± 3.6 versus 17.3 ± 3.5; P ≤ 0.0001) and more total-born piglets (11.9 ± 1.8 versus 10.6 ± 2.0; P = 0.03).

Our data show that primiparous sows nursing small litters exhibit a shorter WEI, which suggests that follicle growth resumes during late lactation. These data also support our hypothesis that feeding altrenogest (AT) during late lactation suppresses the resumption of follicle growth and results in a normal WEI. If we accept that follicular phases will be of similar length in both treatments, it suggests that the follicular phase in AT-fed sows tended to be initiated 3 d before weaning and, by extension, the AT-fed sows were subject to approximately 3 d of follicular growth suppression. Previous studies with weaned sows have shown that a short duration of AT feeding can result in poorer performance in sows that had large follicles at weaning, probably because these follicles lose their estrogenic capacity and become atretic (6). In the present study, however, this adverse effect of short-term AT feeding on fertility was not evident when the ovaries had small follicles at weaning, as would be expected in late lactation.

It has been demonstrated that follicles continue to grow to about 4 to 6 mm even under AT suppression (5,9). Therefore, it is apparent that, while AT suppresses both luteinizing hormone (LH) and follicle-stimulating hormone (FSH), the degree of suppression, particularly for FSH, is not sufficient to prevent follicles from growing to a medium size and sufficient gonadotrophic activity remains to allow limited follicle growth (5). This may explain, at least in part, why the feeding of AT to gilts results in increased ovulation rates and larger litters for gilts and young sows (10–12). In a scenario analogous to the mechanism of estrus synchronization, at the initiation of AT feeding, the growth of medium follicles is arrested, while smaller follicles grow to medium size. When AT is withdrawn, the follicle pool would contain the original medium follicles and other small follicles that had time to achieve medium size. If these follicles have not initiated apoptosis and oocyte degeneration, an increased ovulation rate and potentially larger litters will ensue.

In conclusion, this study showed that primiparous sows nursing small litters are likely to initiate a follicular phase during lactation, which leads to shorter and unpredictable weaning-to-estrus intervals (WEIs). The feeding of AT prevented this follicular phase during lactation, normalized the WEI, and increased ovulation rate and subsequent litter size.