April, 2000 . Volume 23, Number 3 
NCSU Extension Swine Husbandry
April, 2000 . Volume 23, Number 3
Authors' Note: This article is the first in a series on managing the effects of seasonal infertility on sow farms. Next month's issue will offer specific management strategies to combat the heat stress problems described in this issue of Swine News.
Introduction
During summer and early fall, many sow farms experience a variety of reproductive problems.
These problems can be, but are not limited to, anestrus, extended or no weaning-to-estrus
intervals, poor conception rates, and higher embryo mortality rates (low farrowing rates).
This phenomenon is commonly referred to as "seasonal infertility" and can also
be present in boar studs, where the result is lower semen output and lower semen quality.
Most researchers have attributed this seasonal infertility to two factors—heat stress
and the fact that pigs are inherently seasonal breeders. Before pigs were domesticated, both
sows and boars were seasonal breeders. In North America, pigs entered an anestrous season
in the summer and early fall, which avoided the birthing of offspring in winter, a time when
survival would not be optimal.
Even though numerous factors may influence sow fertility on a sow farm today, management and
environment appear to be the two most influential ones. This paper will address these two
factors and provide the most current, effective practices to reduce the impact of seasonal
infertility.
The Effect of Heat Stress and "Season" on Fertility
Much like humans, pigs feel heat based on temperature and humidity. Sows and boars both
suffer from acute and persistent exposure to elevated ambient temperatures and humidity. As
a result, infertility can be relatively short term or, sometimes, a disability from which
the animal will never recover. In most cases, the effect of heat stress on reproduction has
been related to ambient temperatures in excess of 800 F. In studies where
temperatures were elevated experimentally for sows and gilts, anestrus increased and
conception rates and embryo survival decreased.
There is also evidence that boars exposed to ambient temperatures in excess of
850 F have lower sperm output and lower sperm quality. Normally, if this exposure
is short, recovery will occur in 6 to 7 weeks. But it takes 6 to 7 weeks for a new group of
sperm cells to mature within the boar's testicle and epididymis, and the immature,
developing sperm cells tend to be more sensitive than the mature ones. If a boar is
subjected to heat stress sporadically over a two-month period, it is possible that the
fertility of his semen could be reduced during this period as well as for the 6- to 7-week
period after it has ended.
To determine if pigs are feeling the effect of heat, respiration rate can be monitored when
they are in a resting, non-agitated state. This can be done by counting the number of times
the rib cage moves in and out in a minute. Normal respiration rates for pigs are between 15
and 25 breaths per minute. When respiration rates exceed 40, the pigs are at risk of heat
stress. If respiration rates climb over 60, then the pigs are probably suffering heat stress.
Under normal conditions, animals are in physiological homeostasis—in other words, all
bodily systems are functioning normally. Heat stress poses a physiological challenge to
breeding females, and consequently, to reproduction, because their bodies are programed
first to survive and then to reproduce. It is difficult to assess the physiological status
of an animal and determine at what point it will return to reproductive readiness after a
period of stress. Each animal may be at a different level of responsiveness during high
temperatures. However, we know from experience that in the fall, when environmental
conditions return to "normal," a consistent and regular estrus returns. Some of
the management strategies outlined below will be more effective than others. Also, each
strategy's effectiveness can be highly variable because females in a heat-stressed herd will
be at different levels of reproductive competence. In short, some females may require
greater levels or duration of intervention to return to normal reproduction.
Ventilation and Cooling
The first step in reducing the impact of heat stress on sow fertility is to make sure
ventilation systems are in good working order and are providing adequate air movement.
Ventilation rates for a sow and litter, gestating sow, and breeding barn sow during
the summer months are 500, 150, and 300 CFMs/hd, respectively. Following a thorough
maintenance inspection, test the ventilation system to ensure that these rates are met.
It is not uncommon to find that even in fairly new operations, ventilation systems do
not operate as designed. Unless these systems are delivering the required ventilation rates, other
management practices suggested here will not be effective. Additionally, fresh air must
enter rooms at a speed of 600-1,000 ft/min. in order to circulate well and and prevent cold
air from falling on animals (drafts). Don't overlook the fresh air inlets; adjustments
should be made seasonally. A good, year-round air-inlet-speed-goal is 900 ft/min.
Pigs are more sensitive than humans to the combined effects of heat and relative humidity
because they do not sweat. Thus, it is important to consider heat indexes and to adjust the
activation temperatures of supplemental cooling systems. For example, if it is
750 F in the barn, but the heat index is over 850 due to high
humidity, the supplemental cooling system needs to be active. It is imperative that
supplemental cooling systems are in place in all phases of sow production. These could
include evaporative drip or spray cooling and circulating fans. Sprinkling is preferred to
fogging, which uses smaller water droplets. Sprinkling cools the skin surface by wetting
the skin and allowing the water to evaporate, where fogging cools the air and then the
air must cool the skin. Most systems will be designed to operate for a period of 1 to 2
minutes up to 4 times per hour. Spray nozzles should provide at least 0.02 gallons of
water per hour per head. Low-pressure drip systems in the farrowing house should be rated
for 0.5 to 1 gallon per hour.
Most operations in the southern states also have installed components such as cool cells,
which can be effective in keeping room temperatures 10 to 15 degrees cooler than outside
temperatures. Another effective method to cool sows during lactation is the installation
of nose coolers. In farrowing rooms equipped with negative pressure systems using a plenum
as the air inlet source, a tube can be connected to the plenum and directed to the bottom
of the farrowing crate near the sow's nose while she is lying down. This supplies constant
air movement across her face when the ventilation system is activated.
The activation temperature for these systems should be set between 750 and
780 F, as opposed to between 800 and 850. While this
practice may not be able to maintain a farrowing room at between 750 and
780 if the ambient temperature outside reaches 900 +, it will keep the
room from heating up as quickly, since the cooling process will begin sooner.
Along with earlier activation of the cooling systems, replacing heat lamps with regular,
"household," 100-watt incandescent bulbs will reduce the ambient temperature of
the farrowing room. Furthermore, heat lamps may need to be shut off completely during
periods when temperatures do not fall below 850 F to help reduce room
temperatures. If this strategy is practiced, a source of light will be needed somewhere in
the room because some producers report lactation failure if sows and litters are subjected
to total darkness on a continual basis.
Periods of elevated temperatures also can harm the gilt pool. It is not uncommon to see
increased periods of anestrus, shorter estrus periods, and lower conception rates in gilts
during this time. In controlled studies, when higher temperatures were found to induce
anestrus in gilts, cyclicity resumed after exposure for as little as 1 to 2 days to a
relatively normal thermal environment. It may be possible to utilize this concept on
commercial farms by constructing a "cool zone" in the breeding barn for the gilts.
The minimum period that gilts will need to be exposed to this environment to resume
cycling has not been determined, which means that if 2 to 3 weeks are required, a fairly
large area in the breeding barn may be needed. Furthermore, cold water and air movement
may not be sufficient cooling mechanisms when one considers humidity to be an equal
contributor to heat stress. Consequently, some type of air conditioning system may be
needed to remove humidity. The cost of this type of system not be may justified unless there
are extensive problems with anestrus (< 10% cycling).
Changing Photoperiods
Aside from the studies on humidity and temperature, other research on sows bred in the
summer and early fall has shown that gradual changes in the length of the day (photoperiod)
may possibly reduce fertility. These studies looked at pigs kept in a constant,
thermoneutral environment and measured fertility by farrowing rate and litter size. But the
stimulatory influence of photoperiod on sow reproductive performance has not been
consistently proven, and most results from these studies suggest that temperature has a
greater effect than photoperiod. Reducing the photoperiod (10-h light/ 14-h dark) during
high ambient temperatures in most studies did restore good estrus intervals. Some studies
from Australia suggest that shortening the photoperiod is stimulatory, while other studies
indicate the opposite effect. There may be several valid reasons for these differences.
First, changes in photoperiod, in nature, are gradual, not acute, and occur over a long
period. Most studies that have been conducted both in controlled situations and on farm
probably have not employed a long enough period of exposure to an altered photoperiod to
actually see an effect, if one was present. The stimulatory aspect of photoperiod may be
related to the actual ratio of light to dark; however, the rate of change or the time
period over which the change occurs may be equally important, and, therefore, a year-round
schedule of equal light and dark periods may not be of benefit.
Production Scheduling
Season of the year, disease, environment, age, and genetic makeup influence the number of
females showing estrus and conceiving at a particular time. The number of replacement gilts
needed to complete a farrowing group must be determined in advance. When figuring the number
of replacement gilts needed, selection of as many as three for each farrowing crate to be
filled may be necessary. During hot weather, the number of gilts needed to insure one
pregnant gilt at the desired time doubles or even triples. The more gilts in the pool at
any one time, the greater the chance of obtaining more than enough pregnant females for
a predetermined schedule. However, space in the gilt pool is often allotted based on the
average annual need. Increasing the number of available females without simultaneously
increasing space allowance most likely will result in additional stress on the gilts through
crowding, which may ultimately increase the incidence of anestrus.
Sow Mortality
It is not uncommon to see typical herd mortality rates of between 5 and 10 percent or even
greater in many southeastern operations. In many operations the bulk of sow mortality
occurs within the first 30 days following parturition. Furthermore, it is not uncommon for
sow mortality to double in the Southeast during the summer months because heat stress is
added to this already stressful condition of farrowing and lactation. There are no specific
interventions to lower the incidence of mortality during periods of elevated temperatures.
Ultimately, effective management strategies that reduce the effects of heat stress on
reproduction, which will be discussed in next month's issue, should indirectly reduce the
mortality rates as well. Genetic selection for longevity should also be a priority It cannot
be overlooked, considering that the swine industry as a whole has tended to ignore this
trait in order to make rapid gains in the selection for leaner, later-physically-maturing
genotypes.
Kevin Rozeboom
Todd See
Billy Flowers
The University of Minnesota Veterinary Diagnostic Laboratory recently isolated what is
believed to be the first European-like strain of porcine reproductive and respiratory
syndrome virus (PRRSV) identified in North America. The virus was detected in tissuesand
serum from pigs in a herd that had had a mild episode of late-term abortions and weak born
pigs. The herd was previously a PRRSV-positive herd that had been vaccinated. Sequencing of
a region of the viral genome demonstrated approximately 95 percent similarity to the
Lelystad virus, with less than 60 percent similarity to previously known North American
isolates of PRRSV. The strain is being called "European-like" because while it is
similar to the Lelystad virus there are significant differences. Lelystad virus is the
prototype PRRSV in the European Union.
More than 500 PRRSV isolates collected from herds nationwide have been sequenced at the
University of Minnesota over the last five years. All have been classified as North American
strains. While this is the first known detection of a European-like PRRSV, it is unknown if
this strain has been in the U.S. previously. Epidemiological investigations in the herd are
still in progress and may provide some insight.
Currently, there are no commercially available serologic tests in the U.S. to differentiate
antibodies with exposure to European-like strains of PRRSV from those of North American
strains.
The commonly used IDEXX ELISA will detect antibodies to this strain and North American
strains. Further study is needed to determine the ability of other serologic tests to
detect the European-like PRRSV. Differential ELISA tests could be developed. To identify the
European-like virus, it must be isolated and then genetically sequenced. The European-like
strain of PRRSV grew on alveolar macrophages, but not MARC cells, so diagnostic laboratories
using only MARC 145 cells might not detect European-like PRRSV.
This finding has implications for producers and practitioners in the diagnosis and control
of PRRS. With regard to protection by currently available vaccines, the Ingelvac® PRRS
(Boehringer Ingelheim Vetmedica, Inc.) vaccine has been shown to be partially protective
for the Lelystad virus. We do not have information about cross-protection provided by other
PRRS vaccines. Herds that are negative for all strains of PRRSV should continue procedures
to keep the virus from entering the herd. PRRSV-positive herds may want to identify PRRSV
strains within their herd and PRRSV strains in source herds. It is possible that
recombination of the European-like PRRSV and other strains could occur. This would further
increase the heterogeneity of the PRRSV strains in the U.S., which may reduce protection by
naturally acquired immunity and the efficacy of available vaccines.
American Association of Swine Practitioners