While the causes of seasonal infertility in swine are still being debated, most veterinarians agree that high barn temperature and high humidity are the primary culprits.



High temperature and humidity lead to lower feed intake during lactation and heat stress in animals unable to maintain normal body temperature by panting or sweating. This produces a myriad of fertility problems. These include non-cycling gilts, delayed returns to estrus, early embryonic death, irregular returns to estrus, loss of pregnancy, lower litter size and lower farrowing rates.

This summer slump in fertility is one of the most costly problems producers face. It results in lower numbers of weaned pigs per week and less efficient facility use.

To alleviate high temperature and humidity causes of summer infertility, improve animal comfort and increase nutrient intake, especially during lactation. Drippers, snout coolers, evaporative coolers, fans and even air conditioning can boost sow comfort.

The most reliable way to counter the low farrowing rate is to simply breed more females during those months that produce the lowest farrowing rates. The following case study is an example of how one producer offset the summer slump, by planning and looking at past herds' reproductive performance.

Table 1. Weekly Breeding Targets Based on Breed Month
% *
Jan 81 80
Feb 80 81
Mar 78 83
Apr 76 85
May 77 84
Jun 78 93
Jul 68 96
Aug 66 98
Sep 70 93
Oct 77 84
Nov 84 77
Dec 83 78
% Historical farrowing rate (last 3-year average)
* Weekly breeding targets to farrow 65


A 1,400-sow commercial producer called in February about the low number of sows farrowed in the last few months and the lower number of pigs weaned. First concerns were for the health of the sow herd. Specifically, he wanted to know whether to make changes in the porcine reproductive and respiratory syndrome (PRRS) control strategies started the year before. Sow farrowing rate in November and December had dropped below 70%. The producer knew there was a hole in his throughput that would be most noticeable next summer when he would have fewer market hogs.

We used the production record system to rule out an infectious cause. What we discovered was that for the previous three years the farm had experienced a similar fertility problem with summer matings (Fig. 1). Using the information generated from the production records, we could easily calculate the number of females that would have to be serviced weekly to maintain optimal throughput during the summer months of lower fertility. We then generated a table that directed the breeding manager to adopt weekly breeding targets based on the breeding month (Table 1).

Also, immediate action was taken to make more replacement gilts available for mating during the peak needs months. Accommodations were found for the additional inventoried gilts and gestating sows.

At the same time, other measures to counter the summer slump such as those mentioned above were instituted.

As a result of these actions, the breeding manager was able to meet or exceed the new breeding targets about 80% of the time during the following year.

Although the farm still experienced the summer slump, the increased breeding targets allowed them to maintain throughput and actually wean a record number of pigs for a calendar year.