Low Feed Intake During Lactation Negatively Impacts the Weaning-To-Estrus Interval in Sows

Livestock Update, October 2003

Mark Estienne, Swine Research Physiologist, Ryan Horsley, Graduate Student, Allen Harper, Extension Animal Scientist-Swine, Tidewater AREC

Introduction
Many factors influence the overall reproductive efficiency of a swine operation. One example is the average number of sow non-productive days (NPDs). NPDs accumulate for sows that are not pregnant or lactating. In other words, sows cannot be considered "in production" unless they are carrying or nursing a litter. For reproductive efficiency to be maximized, NPDs must be minimized.

In modern breeding units, it is expected that sows return to estrus or "heat" within seven days after weaning. For sows that do not exhibit estrus promptly after weaning, more NPDs accumulate, thus compromising overall breeding herd performance.

There are numerous factors that influence the rapidity with which sows return to estrus after weaning. This article focuses on the effects of low feed intake during lactation on the weaning-to-estrus interval.

Reproductive Endocrinology of the Lactating and Weaned Sow
The pituitary gland is a garden pea-sized organ that is located just below the brain in swine and other mammals. Numerous hormones are secreted into the circulatory system from the pituitary gland, including luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH and FSH are referred to as gonadotropins and cause follicles to grow on each of the two ovaries. Follicles secrete estradiol into the circulation, which occasions the behavioral and physiological changes associated with estrus.

During lactation, suckling by pigs causes a suppression of LH and FSH secretion and as a consequence, blood concentrations of the gonadotropins are generally low in postpartum sows. Between days 14 and 21 post-farrowing, however, release of LH and FSH into the circulation increases and this increase continues until weaning. The removal of the suckling stimulus at weaning allows a further increase in gonadotropin secretion. Follicles rapidly grow and there is a corresponding rise in circulating levels of estradiol. Sows return to estrus and the estradiol triggers a massive release of LH which results in the process of ovulation (release of eggs from follicles).

Effect of Feed Intake on the Weaning-to-Estrus Interval
Several researchers have demonstrated that sows consuming low amounts of feed during lactation have a delayed return to estrus after weaning. For example, Pettigrew and King (1992) conducted an experiment during which sows were fed various amounts of feed during a 28-day lactation. The percentage of sows displaying estrus within 8 days after weaning was 8%, 33%, 50%, 58%, 58%, and 83%, for females consuming 3.3 lbs., 4.9 lbs., 6.4 lbs., 7.9 lbs., 9.3 lbs., and 10.6 lbs. of feed daily during lactation, respectively.

At the Tidewater Agricultural Research and Extension Center in Suffolk, we monitored daily feed consumption in a group of first- and second-parity, crossbred sows (n = 17 sows) during an 18-day lactation. Sows were nursing an average of 9.5 pigs. The interval from weaning to the onset of estrus was negatively correlated with daily feed intake during lactation (R2 = -0.44; P = 0.07). Figure 1 illustrates the inverse relationship between daily feed consumption and the weaning-to-estrus interval.

Figure 1. Relationship between feed consumption during lactation and the weaning to estrus interval in sows.

The delay in the return to estrus after weaning in sows that consumed low levels of feed during lactation in all likelihood involves a suppression of gonadotropin secretion. Tokach et al. (1992) reported a positive relationship between energy intake and LH secretion in sows as early as 14 days after farrowing. In their study, sows that returned to estrus in less than 9 days after weaning on Day 28 of lactation had higher circulating concentrations of LH at 14, 21, and 28 days postpartum compared with sows that required more than 15 days to return to estrus.

Factors Affecting Feed Intake during Lactation
Feed intake during lactation is influenced by numerous factors including parity, environmental temperature and level of feeding during gestation.

Parity. Multiparous sows generally consume more feed during lactation than do primiparous sows (Britt, 1996). This could partially explain the fact that multiparous sows respond better to early weaning systems than do primiparous sows. In a retrospective study conducted using 178,519 litter records obtained from 13 commercial herds, Mabry et al. (1996) found that decreasing lactation length had a negative influence on the weaning-to-estrus interval, and this effect was most pronounced in primiparous compared with multiparous sows.

Environmental Temperature. Sows that farrow in the summer and early fall often have an extended weaning-to-estrus interval and the delay is more pronounced in primiparous compared with multiparous sows. Sows lactating during the summer eat less and loose more backfat. Barb et al. (1991) reported lower levels of LH in sows exposed to temperatures of 86º F compared to sows maintained at temperatures of 72º F.

Feed Intake During Gestation. It is well established that the greater the feed intake during pregnancy, the lower the feed intake during lactation. Cole (1989) suggested that when feed intake during pregnancy exceeds 4.4 lbs. per day, feed intake during the subsequent lactation is reduced.

Based on last rib backfat thickness, Estienne et al. (2000) characterized sows at farrowing as fat, medium, or thin. During lactation, thin sows consumed more feed compared with fat and medium sows. In a subsequent study (Estienne et al., 2003), gilts were fed diets during gestation that differed in energy level. During gestation, gilts consuming the high-energy diet gained more body weight and backfat thickness than gilts fed the low-energy diet. During lactation, however, gilts that were fed the low-energy diet during gestation consumed more feed than gilts fed the high-energy diet during pregnancy.

Management Considerations
In order to minimize the weaning-to-estrus interval, it is critical that a high level of feed intake be achieved by the sow during lactation. The following are some key considerations for managers and herdsmen:

• Care should be taken not to over-feed gestating sows. This will help circumvent a voluntary reduction in feed intake during lactation that results in considerable body weight loss and condition.
• Feeding lactating sows two or more times daily compared to once daily maintains feed freshness and promotes greater consumption.
• Always allow sows unlimited access to clean water to promote greater feed consumption. Nipple waterers should have a minimum flow rate of 1 quart per minute (15 seconds to fill an 8-ounce cup).
• Manage farrowing rooms to optimize sow comfort. The thermo-neutral range for lactating sows is approximately 55 to 75º F. During the summer, sows should be kept cool in order to stimulate an increase in consumption of feed. Consider installing drip cooling or other types of cooling systems.
• For sows that display poor feed intake during lactation, consider treating them with P.G. 600 at weaning time (see section below).

Use of P.G. 600 to Counteract the Negative Effects of Low Feed Intake during Lactation on the Weaning-to-Estrus Interval

The active ingredients of P.G. 600 (Intervet America Inc.; Millsboro, DE) are pregnant mare serum gonadotropin (PMSG) and human chorionic gonadotropin (hCG), two hormones that mimic the actions of FSH and LH, respectively. Thus, when P.G. 600 is administered to weaned sows, follicular growth is advanced, and estrus and ovulation follow.

In research conducted on commercial farms, scientists demonstrated that administration of P.G. 600 to first and second parity sows at weaning decreased the days to estrus. Treatment with P.G. 600 also lowered the percentage of first litter sows not exhibiting estrus within 10 days after weaning (Bates et al., 1991). Estienne and Hartsock (1998) reported that treatment with P.G. 600 increased the percentage of multiparous sows in estrus within 7 days after weaning, and relative to controls, sows given P.G. 600 expressed estrus sooner. In both of these experiments (Bates et al., 1991; Estienne and Hartsock, 1998) sows were allowed ad libitum access to feed.

At the Tidewater Agricultural Research and Extension Center, we recently conducted an experiment to test the hypothesis that P.G. 600 could counteract the negative effects of low feed intake during lactation on the weaning-to-estrus interval in sows.

First and second parity, Yorkshire x Landrace sows nursing approximately 9.8 pigs each were utilized. During an 18-day lactation, sows were fed on an ad libitum basis (n = 18 sows) or were restricted to 7 lbs. of feed per day (n = 35). The restricted-fed sows were used to mimic sows displaying poor appetite during lactation. The lactation feed employed met or exceeded National Research Council (NRC, 1998) recommendations for the various nutrients and sows were allowed ad libitum access to water.

Table 1 displays sow body weights, last-rib backfat thickness as determined ultrasonically, and feed consumption for sows fed on an ad libitum or restricted basis. As expected, restricted-fed sows lost more body weight and backfat during lactation compared with ad-libitum fed sows. Moreover, characteristics of the restricted-fed sows were similar to those commonly displayed by sows consuming low levels of feed during lactation, validating our experimental model.

Table 1. Characteristics of sows fed on an ad libitum or restricted (7 lbs./day) basis during an 18-day lactation.

Ad Libitum

Item:	Restricted	SE	P
No. Sows Body Weight (lbs.)	18	35	---	---
Farrowing (F)	462.6	455.6	9.2	NSa
Weaning (W)	456.9	406.7	9.1	<0.01
Change (F ­ W) Backfat (mm)	-5.7	-48.9	5.2	<0.01
Farrowing (F)	18.0	16.7	0.9	NS
Weaning (W)	17.2	13.4	0.8	<0.01
Change (F ­ W)	-0.6	-3.1	0.6	<0.01
Feed Consumed (lbs./day)	11.7	6.8	0.1	<0.01
aNS = non-significant difference.

At weaning, sows fed on an ad libitum basis (n = 18 sows) were injected i.m. with 5 mL deionized water. Restricted-fed sows were treated i.m. with deionized water (n = 19 sows) or P.G. 600 (n = 16 sows).

The percentage of sows displaying estrus by day 7 after weaning was higher for ad libitum-fed sows injected with deionized water compared with restricted-fed sows injected with deionized water (Table 2). The percentage of P.G. 600-treated, restricted-fed sows displaying estrus by day 7 post-weaning was intermediate between the other two groups. For those sows exhibiting estrus by day 7 after weaning, there was no effect of treatment on the weaning-to-estrus interval.

Sows were slaughtered at Day 30 post mating and reproductive tracts were examined. There were no statistically significant differences between groups for pregnancy rate, ovulation rate or number of live embryos (Table 2).

Our data suggests that in situations where sows are consuming low quantities of feed during lactation, P.G. 600 can be used to decrease the percentage of females not in estrus within 7 days after weaning. Although in the current experiment we did not treat any ad libitum-fed sows (consumed over 11 lbs./day) with P.G. 600, the high level of post-weaning performance in this group suggests that P.G. injection would only be warranted in sows with low feed intake during lactation.

Table 2. Post-weaning reproductive performance in sows fed on an ad libitum basis during an 18-day lactation or allowed 7 lbs. feed/day during lactation (restricted) with or without P.G. 600 at weaning.

Item:	Ad libitum	Restricted	Restricted + P.G. 600	SE
No. sows	18	19	16	---
Estrus by day 7 post-weaning (%)	94.4a	68.4b	87.5a,b	---
Weaning-to-estrus (days)c	5.3	5.3	5.0	0.3
Pregnant at day 30 post-mating (%)	88.2	92.3	71.4	---
Ovulation rate	17.2	16.5	18.3	1.3
Live Embryos at day 30 post-mating	11.0	10.3	10.6	1.2
a,b Within rows, means with different superscripts differ (P < 0.05). c Includes data for sows displaying estrus < 7 days post-weaning only.

References
Barb, C.R., M.J. Estienne, R.R. Kraeling, D.N. Marple, G.B. Rampacek, C.H. Rahe, and J.L. Sartin. 1991. Endocrine changes in sows exposed to elevated ambient temperature during lactation. Domestic Animal Endocrinology 8:117.

Bates, R.O., B.N. Day, J.H. Britt, L.K. Clark, and M.A. Brauer. 1991. Reproductive performance of sows treated with a combination of pregnant mare's serum gonadotropin and human chorionic gonadotropin at weaning in the summer. Journal of Animal Science 69:894.

Britt, J.H. 1996. Biology and management of the early weaned sow. Proceedings of the American Association of Swine Practitioners 27th Annual Meeting pp.417-426.

Cole, D.J.A. 1989. Sow nutrition- The key to profitable pig production-more piglets in less time. Biotechnology in the Feed Industry- Proceedings of Alltech's Fifth Annual Symposium. pp.107-120.

Estienne, M.J., and T.G. Hartsock. 1998. Effect of exogenous gonadotropins on the weaning-to-estrus interval in sows. Theriogenology 49:823.

Estienne, M.J., A.F. Harper, C.R. Barb, and M.J. Azain. 2000. Concentrations of leptin in serum and milk collected from lactating sows differing in body condition. Domestic Animal Endocrinology 19:275.

Estienne, M.J., A.F. Harper, D.M. Kozink, and J.W. Knight. 2003. Serum and milk concentrations of leptin in gilts fed a high- or low-energy diet during gestation. Animal Reproduction Science 75:95.

Mabry, J.W., M.S. Culbertson, and D. Reeves. 1996. Effects of lactation length on weaning to first service interval and first service farrowing rate in commercial sows. Swine Health and Production 4:185.

NRC. 1998. Nutrient Requirements of Swine. 10th ed. Natl. Acad. Press, Washington, DC.

Pettigrew, J.E. and R.H. King. 1992. Modern sows need nutrition to match. National Hog Farmer 37(10):40.

Tokach, M.D., J.E. Pettigrew, G.D. Dial, J.E. Wheaton, B.A. Crooker, and L.J. Johnston. 1992. Characterization of luteinizing hormone secretion in the primiparous, lactating sow: Relationship to blood metabolites and return-to-estrus interval. Journal of Animal Science 70:2195.

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