As the day length starts to shorten after a summer of heatwaves, what effect is this going to have on fertility?
Seasonal infertility is the reduction in fertility seen in late summer and into the autumn. It is a major issue for the pig industry worldwide, with US data suggesting it costs $300 million/year. The two main causes for this phenomenon are heat stress and photoperiod.
Heat stress occurs when the pig is producing and receiving more heat from its environment than it is releasing. This causes a series of metabolic and physiological effects in both sows and boars in order to keep cool. Feed intake will be reduced which limits the nutrients available, leading to the mobilisation of body reserves to tackle the deficiency. In sows, this nutrient deficiency will have negative impacts on the production of reproductive hormones (luteinising hormone) altering the development of follicles in the ovaries, resulting in reduced conception and farrowing rates. In boars, libido is reduced and high temperatures have a destructive effect on germ cell development, causing lower numbers and more abnormal spermatozoa. It can take up to 8 weeks for sperm motility to return to normal after a hot spell.
The impact of heat stress can be greatly reduced through management to keep the sows cool and optimise feed intake. This can be achieved by feeding at cooler times of day, providing shade and also areas such as mud wallows to assist in evaporative cooling. Clean, fresh drinking water is also essential, cool if possible. The above will have more impact than feed changes, but reducing the fibre content in the diet will reduce the thermal effect from fibre fermentation in the hindgut.
Following the reproductive issues with hot weather; photoperiod is the key autumnal cause of infertility as the hours of daylight reduce. Being descendants of wild boar, which are seasonal breeders, pigs do not want to produce offspring in the winter when there is decreased availability of food. This is a trait which hasn’t been fully bred out of modern genotypes. Providing artificial light to ensure 16 hours of photoperiod, with a minimum 200 lux at pig level, can help to prevent an increase in the weaning to service interval. The swings between day and night temperature also causes issues. Extra energy in the diet may be needed to overcome this and maintain body temperature above the lower critical temperature, which is 16°C for sows.
In summary, seasonal infertility is a common seasonal issue faced by the pig industry. The heat of the summer followed by shortening days of the autumn provides a challenge in maintaining fertility levels. Thorough management is required to ensure that pigs are kept within their thermal optimal zone, allowing them to optimise their nutrient intake and, therefore, maintain steady production.