NCSU Extension Swine Husbandry 2000
A more printable version of Swine News in Adobe Acrobat.

May, 2000 . Volume 23, Number 4
April Swine News


Author's Note: This is the second part of an article on managing the effects of seasonal infertility on sow farms. This month's issue deals with specific management strategies to combat the effect of heat stress on reproduction, which was discussed in the previous issue of Swine News.

Feed Bunk Management

Without question, getting sows to keep eating as much as they need or to prevent a dropoff in their consumption during the summer months is the most critical management step for reducing the impact of heat stress on seasonal infertility. Here are some ways to accomplish it:

Increase Feeding Frequency. When producers switch from feeding two times per day to three times per day, most experience a 10 to 15 percent increase in sow feed intake. There are some farms in North Carolina that actually feed four or more times per day in the summer. The main thing to remember is that when you increase the frequency of feeding, you must decrease the amount that you feed each time. For example, if you are feeding 6 pounds twice a day (12 pounds total), then when you increase to three times per day, you may want to feed around 4.5 pounds at each feeding (13.5 pounds total).

The reason this strategy works is related to the normal increase in body temperature that occurs after a sow consumes a meal. Theoretically, there won't be as big an increase in a sow's body temperature after she eats 4.5 pounds (as after she eats 6 pounds) because there will be less feed to be digested. Consequently, this could be very important for sows whose body temperatures already may be in the upper end of the thermoneutral range due to high temperatures in their environment.

Keep Feed Fresh. Sows tend to be picky eaters compared to most animals (contrary to popular belief). In warm conditions, feed is more likely to spoil, especially if it contains high levels of fat. Increasing the feeding frequency in conjunction with feeding slightly smaller meals is an excellent way to keep feed fresh.

Try Liquid Diets. Liquid feeding is a common practice to increase feed intake in many finishing operations and can be implemented during lactation. However, because of the short period of time that sows are actually in lactation, it may be more beneficial to acclimate females to this change of diet during late gestation. Success with this strategy may vary greatly among operations, but it has been reported to boost sow feed intake by as much as 15 percent. One drawback is that wet feed does not stay fresh in the trough for very long.

Add Fat to the Diet. As a result of poor feed intake, many sows are not able to meet the metabolic demands of lactation and may fall into a negative energy balance. This factor probably accounts for most of the reproductive disorders during periods of elevated temperatures. One way ensure that sows are consuming enough energy, even though they are eating a smaller quantity of feed, is to add fat to the lactation diet. Supplemental fat (7 to 10 percent animal or vegetable fat) will increase the dietary metabolic energy content of the feed.

There are two important considerations in adopting this practice. First, a diet containing high amounts of fat will become rancid more rapidly than a traditional diet with only 1 to 2 percent fat, and sows will not eat rancid feed. Therefore, feeding smaller quantities more often and smelling leftovers in the sow feeder at each feeding to check for spoilage should be a standard practice. Second, because sows are consuming less feed, dietary levels of essential vitamins and minerals also need to be boosted to compensate for less feed consumed on a daily basis.

Give Water Constantly. High ambient temperatures will increase water requirements, particularly for sows. Increased water consumption coupled with increased urinary water loss is one mechanism by which pigs lose body heat. An increase in temperature from 54°-60° F to 86°-95° F will lead to pigs' drinking more than 50 percent more water. Nursing sows need to consume 8 to 10.5 gallons of water every day, and gestating sows need 3 to 5 gallons. One rule of thumb to follow is a water-to-feed ratio of 5:1. Fresh, constant water is also critical during breeding and gestation. The watering system should deliver a minimum of 0.25 gallons per minute and ideally 0.5 gallons per minute. Sows will quickly become frustrated if the flow rate is low, and this will reduce their appetite for dry feed. Water temperature and quality are also important. During periods of high temperatures, pigs will consume almost double the quantity of cool water (50º F) as warm water (80º F).

Reducing Embryo Mortality During Periods of Heat Stress

Prenatal mortality may be as high as 40 percent in pigs. The bulk of this embryo loss occurs during the first two to three weeks following breeding. Factors associated with embryo loss include stage of pregnancy, disease, age of dam, genetic factors, nutrition, external environment, intrauterine environment, and stress—including heat stress. When pigs are under extreme heat-related stress during the first 30 days following breeding, it is imperative that the following recommendations be put into effect to avoid increased embryo mortality:

(1) avoid late estrual inseminations,

(2) minimize unnecessary stress by mixing females only at weaning,

(3) refrain from or even stop moving females in gestation to different locations, and

(4) provide a good, level plane of nutrition following during and after breeding. The strategies also should be used through the year.

Late Insemination. Several processes occur following breeding to optimally prepare the uterus for implantation. A postbreeding inflammatory response occurs in the uterus of the pig in the removal of nonfertilizing spermatozoa and bacteria. In addition, during early to mid-estrus, uterine contractions help physically to remove the products of this inflammation. The first step in limiting embryo loss should be taken during the estrus period, and that is to avoid late inseminations. The simplest way to prevent late estrual inseminations is to ignore the "target" number of inseminations and breed females totally on the basis of a strong, standing heat response. Another way to reduce mistimed inseminations is to determine the average estrus length in your weaned sows, gilts, and repeat breeders and based on these averages, shorten the last insemination interval up. For example, if you normally service sows AM-AM-AM, change your schedule to AM-AM/PM. Thorough heat-checking before performing subsequent inseminations also will help prevent poorly timed, late artificial inseminations, which may interfere with uterine preparation for implantation.

Mixing Females. Once fertilization occurs in the oviducts, pig embryos descend into the uterus very quickly. However, implantation does not occur until day 13 and full attachment not until day 18. During this time, the pig is highly susceptible to stress factors, such as movement and temperature. If females are to be mixed, this should be performed on the day of weaning to prevent unnecessary stress on the female. Any unnecessary stress following breeding can result in embryo detachment and loss.

Moving Females. After breeding and around day 30 of pregnancy, females may be moved to a different location; however, mixing sows and gilts at any time during or following breeding greatly increases the chances of subsequent embryo mortality. Temperature changes also are likely to increase embryo mortality, and during early pregnancy females should be protected from heat or cold in order to avoid unnecessary stress. Make sure that cooling and heating systems are routinely maintained and functional. You may also want to have a backup system in place (i.e., hoses and spray nozzles) in case of equipment failures.

Nutrition. As far as diet is concerned, sows and gilts should be fed enough following breeding to keep them on an even plane at maintenance levels or slightly above. The pre-mating nutritional status appears to be a greater determinant of embryo numbers and survival than post-mating diet in gilts. Using this strategy requires "flushing" them with an extra 1 to 2 pounds of feed during the estrus cycle before mating. This can be attempted for sows as well, though most postweaned sows voluntarily restrict their own feed intake. Another consideration in managing embryo survival rates is this: various studies have indicated that high feed intake during the 30 days following breeding may have a negative impact on swine embryos.

There are no extra measures to take in feeding during periods of heat stress, with the exception of ensuring that the female is consuming feed daily (hopefully around 4 to 5 pounds, depending on diet formulation). Appropriate action to boost appetite may be required, similar to the procedures used during lactation.

Hormonal Strategies

Although extensive research has been carried out on using hormone and vitamin therapy during the pre- and postbreeding periods to increase embryo survival, there are no practical methods available to producers that are cost-justified. However, the following basic tips will help provide a good uterine environment for embryos and reduce the postbreeding stress that leads to embryo loss in pigs.

Historically, gonadotropins and progestens have been used with limited success to improve reproductive performance in swine. Nevertheless, application of these hormones in specific swine management areas has helped reduce the reproductive lag associated with heat stress and negative energy balances during lactation. Hormonal strategies using PG600® (400 I.U. PMSG + 200 I. U. hCG), Regumate® (progesterone), and Estrumate® (prostaglandin) may help counteract the negative effects of heat stress. PG600® can be injected at weaning to stimulate follicular growth, speed return to estrus intervals, and reduce the incidence of anestrus. However, cost is a major limitation, and this approach may show benefits only in herds where extended wean-to-estrus lengths (more than 10 days) or high frequencies of anestrus occur. PG600® is most commonly used to stimulate puberty in 145- to 160-day-old gilts. This is generally very effective and may also be useful during periods of high ambient temperatures to stimulate a first estrus in incoming gilts where cyclicity is suppressed. Some producers treat only problem groups of sows, such as those with low feed consumption during lactation or low parity, to improve the efficiency of this technology. Prostaglandins, which are commonly used to induce parturition, are believed to speed uterine recovery when injected postfarrowing. However, prostaglandin used alone has not reduced the incidence of anestrus or extended wean-to-estrus interval (WEI).

Extended WEIs and anestrus following weaning in parity 1 sows are probably the most noticeable effect of heat stress on reproduction. The combination of heat stress, parturition, lactation, and poor feed intake contribute to poor reproduction in all sows; however, P1 females also have a metabolic demand for growth. One strategy to minimize the impact of heat stress on overall herd reproduction is to adjust female replacement schedules to avoid large numbers of P1 farrowings during July and August. It may also be possible to treat this subpopulation of females with hormonal therapy during lactation and at the time of weaning to stimulate the reproductive system. A single shot of PG600® at the time of weaning has been effective in reducing WEI in sows. However, a recent field report suggests that a vulvular injection of 1/2 cc. of Estrumateâ within 24 hours after farrowing in conjunction with PG600® at weaning may be even more effective at reducing WEI and the incidence of anestrus.

Continual feeding of Regumate®(for 14 days) suppresses follicular growth and estrus until withdrawn. Regumate® usage appears to be useful in estrus synchronization of cycling females (especially gilts) and may be a useful strategy to improve reproductive performance after a short lactation in sows (feed the hormone throughout lactation and withdraw at weaning). In this situation, Regumate® is fed for 14 days and followed by an injection of prostaglandin on the morning of day 15. But cost and the delivery system are major limitations of this regimen, especially if sows are not consuming feed during periods of heat stress. Regumate® is currently not approved for swine use and is produced in an oil-base form that is difficult to handle.

Lactation Lengths and Split Weaning

The greatest metabolic demand on the sow is during lactation. Many postweaning reproductive abnormalities can be attributed to low feed intake and depletion of fat reserves in the lactating sow. Possible strategies to reduce nutritional requirements and maintain body condition include a shorter lactation period, cross-fostering to balance litter size, and split weaning. Summarized records from Mabry and Culberston (1998) showed that parity 3+ sows can be weaned, recycle, and conceive efficiently at lactation lengths as short as 9 days. However, first and second parity sows appear to need lactation lengths of 14 and 12 days, respectively, to cycle and conceive efficiently.

Reducing litter size during the last one third of lactation by means of split weaning can be an effective strategy for conserving the body fat stores of the sow. However, some studies suggest the removal of more than two to three piglets earlier than 3 days prior to weaning may cause sows to cycle while still in the farrowing crate. Short cycling can be avoided and split weaning accomplished by removal of the heaviest two or three piglets 3 days before weaning.

Genetic Components

Simultaneous advances have been made in pigs/sow/year, predominantly due to management, and lean (growth rate and percentage), predominantly due to genetics. Today we are managing a more prolific mature sow that may be both leaner and larger in mature body size. In addition, gilts now grow faster, reach puberty at a heavier weight, and are mated both younger and leaner.

Young breeding females of lean genotypes must maintain condition throughout breeding life. This can be accomplished by minimizing lactation fat losses and encouraging gestation fat recovery. Several gilt studies have clearly demonstrated that backfat depths of less than 0.5 inches are associated with reproductive inefficiencies. However, reduced reproductive performance is also consistently reported in females having more than 1 to 1.2 inches of fat when they are introduced into the herd. In short, there appears to be an optimal gilt body condition range for introduction to the breeding herd that is dependent on genetic line. With leaner genetics, there is a substantially increased maintenance requirement throughout lactation; thus management actions to encourage feed intake are needed. The combination of heat stress with substantial loss of body stores in thin females will demand more in the way of reproductive management than a situation involving females with higher appetites and increased fat deposits.

When considering different genetic lines, an important consideration is knowing which performance testing methods and selection objectives were used. It has been well documented that selection for efficient lean growth can adversely affect reproductive performance. Selection for reduced backfat will result in reduced daily feed intake, increased age at puberty, fewer pigs born alive, greater preweaning mortality, and more nonproductive days. Litter size at birth and weaning and piglet weight are also reduced with selection for low daily feed intake. The consequences of today's genetically leaner animal can result in reduced reproductive performance and, most importantly, reduced appetites.

Many of the strategies described throughout this paper will help to alleviate some of the symptoms of seasonal infertility, but a solution to the problem may be in establishing a genetic selection program for females that are more resilient to heat stress. Differences among breeds in the ability to adapt to heat have been reported for cattle, sheep, and goats. The beef cattle industry has perhaps the best documentation that this can be accomplished—its successful incorporation of heat-tolerant breeds (Bos indicus) into traditional crossbreeding systems. In almost all comparative research studies, this crossbreeding strategy has been shown to ameliorate the sensitivity of British and Continental cattle breeds to high temperatures, a sensitivity that manifests itself in higher rectal temperatures, increased respiration rates, and greater decline in milk yield, body weight gain, and feed consumption.


Outlined here are steps to reduce the impact of heat stress (seasonal infertility) on swine reproductive productive performance. In short, all of these considerations must be addressed for producers to cope with reproductive issues associated with seasonal infertility.

1. Incorporate the most advanced methods of ventilation and cooling into all phases of gilt development and sow productivity, and make sure they work. Activate systems earlier to periods of prolonged high ambient temperatures. Keep sows and gilts cool.

2. Schedule animal activities in the early morning and late afternoon, when temperatures are not as extreme, to benefit both employees and breeding females.

3. Set up a Cool Zone (reduced heat index) for the gilt acclimatization.

4. Implement strategies to reduce or avoid sow movements and mixing. Revisit estrus detection strategies to ensure optimal reproductive stimulation and insemination protocols to avoid late estrual inseminations.

5. Evaluate specific problems (i.e., anestrus or prolonged WEI) and apply hormonal treatments when needed as outlined in this article.

6. Feed nursing sows smaller quantities of more nutrient-dense diets more often.

7. Overcompensate for an increase in seasonal anestrus by having 10 to 15 percent more gilts available for breeding, but account for these additional animals, and avoid overcrowding.

8. Implement an early-wean/split-weaning program during the summer that will reduce the metabolic demand on lactating females.

9. Utilize genetics that are less sensitive to heat stress and that have documented track records for superior appetites under normal environmental conditions.

10. Establish a long-term program to reduce the impact of heat stress on the sow herd through genetic selection of animals that can perform well in high temperatures.

Kevin Rozeboom
Todd See
Billy Flowers

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Last modified April, 24 2000.