Boar Fertility Measurements
 
The Power of Boars
 
Although sows and boars are considered equal partners in the breeding process to produce a litter of pigs, each sow produces only two or three litters a year. A single boar usually breeds between 20-1000 sows over the same period, depending on the type of mating (natural hand mating or commercial artificial insemination). Consequently, your decisions regarding sound boar purchases and boar management can have an exponential effect on the farrowing rate and litter size in your herd.

Highly fertile boars are expected to produce large numbers of sperm cells that are capable of fertilizing all the eggs shed by a sow during her heat cycle. Improvements in boar fertility can only be achieved by increasing the number of sperm produced and/or by improving the sperm’s ability to fertilize eggs.
 
Measuring Boar Fertility
 
The best measure of boar fertility is his production of live piglets (farrowing rate x number of pigs born live/litter). However, the short gestation period and early age at sexual maturity of gilts do not easily or economically accommodate this kind of fertility assessment before the boar has bred a number of animals.

A second way of assessing boar fertility is to evaluate semen quality. The three most common qualitative measures performed on semen are:

  1. Sperm cell motility (their ability to swim in a forward direction)
  2. Cell morphology (no crooked tails, deformed heads, dead sperm cells)
  3. Acrosome integrity (cap on the head of the sperm cell which allows penetration of the egg)

Ejaculates with high percentages of motile and morphologically normal cells are of better quality than boars that ejaculate semen with lower percentages. The relationship between the above measurements and semen fertility is not totally clear, however. Although fertility in terms of conception rate and litter size increases as motility increases, there is a point where fertility no longer increases (Table 1).

The critical point for good motility is 60%. The same consideration is given for morphology at 70% and acrosome integrity at 50%. This means that microscopic semen tests group ejaculates into two categories – fertile and subfertile. Of the three factors, acrosome integrity is likely the most sensitive predictor of fertility because of its ability to actually penetrate the egg for fertilization.

A number of other procedures have been looked at to improve the accuracy and precision of fertility tests. Assessment of nuclear structure of DNA in sperm cells, ova penetration, sperm membrane properties and specific proteins in seminal fluid address some of this precision. If the protein in seminal plasma is the most practical measurement, a simple test could be developed for identifying the specific proteins that improve fertility.

Table 1. Relationship between Fertility and Semen Motility

Percentage Motile Sperm

30

40

50

60

70

80

90

100
Farrowing Rate

50%

70%

70%

75%

85%

83%

87%

87%
No. Pigs Born Alive

7.8

8.5

9.0

9.0

10.1

10.5

10.5

11.0
 
Improving Sperm Production
 
A boar may have fertile sperm cells, but if insufficient numbers are ejaculated, then fertility can be reduced.

Positive influences on sperm production:

  • Changes in lighting period may marginally affect sperm cell production
  • A repeatable, highly positive correlation between testicle size and sperm production in both young and mature boars. (Crossbred boars generally have larger testicles than purebreds.)

Negative influences on sperm production:

  • Long periods of under-nutrition. Extreme or prolonged underfeeding is unlikely to occur in well-managed herds. Because sperm production takes 50-60 days in boars, any underfeeding which might have occurred will not be noticed for several months.
  • Exposure to high barn temperatures has immediate, severe and consistent effects, particularly on morphological abnormalities. The minimum exposure time, and upper critical temperatures above which sperm production is impaired, varies according to geographical location.
  • Long-term housing under conditions of temperatures in the upper range of the boar’s comfort zone (26-29 degrees Celsius) may reduce sperm cell numbers and quality. This reduction may be noticed after five or six weeks.
 
Summary
 
A serious limiting factor to improving boar fertility is a quick and accurate test for quality boar semen. Routine semen evaluations under a microscope can be performed to assess semen motility, morphology and acrosome integrity. Don’t use sub-fertile boars as breeders.

The practice of limit-feeding young, growing boars may impair future fertility in these boars. Young boars can be fed 2.3 kg/day of a breeder ration, while mature boars in use will eat 3 kg/day.

Boars with larger testicles should be given selection preference over boars that have small testicles. Increased sperm production results in bigger litters of live piglets.

Boars should never be kept in hot barns. Boars that have been housed under high summer temperatures (including any fevers from illness) will show a lower fertility level about two months after the exposure.
 
 

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