In a pig breeding unit, normally 30% to 40% of sows are replaced by gilts every year. Those gilts are raised in a gilt multiplication unit with a specific health situation and different housing and feeding conditions. The gilts themselves are protected by their herd-specific immunity against the microbes of the multiplier unit where they were born and raised, but not against the pathogens in the piglet producing herd that they enter.

A few years ago, the introduction of gilts into a relatively small herd presented few practical problems. Today, they have to be protected against diseases which impact on fertility. This may be connected with the occurrence of different pathogens such as PRRS or PCV2, in combination with mycotoxins.

When gilts enter the herd, they form antibodies against any pathogens they meet that they have not encountered before. This process takes two or three weeks. During this period, they are either not protected at all or poorly protected against the new pathogens and as a result of that they may become ill, even though there are not always clear symptoms to recognize. Especially in this time, specific vaccinations should be made, leading to a different immunity in gilts depending on the infectious pressure.

Gilts in former times were inseminated during the first estrus after introduction or a maximum of one cycle later. A longer acclimatisational period is necessary today. They should be introduced into the herd with a period of isolation (three weeks) followed by a period of acclimatisation (three or four weeks). As a consequence, modern gilts are purchased a lot younger (some of them under five months old) than a few years ago.

Some weaner producers bunch new gilts into two or three age categories and arrange to introduce them into the herd only six times each year. As a result, pigs aged 166 days and 180 days (or younger) may be introduced together.

A number of problems can occur if the gilts are not introduced effectively. Chief among them is that an uncontrolled contact with the pathogens can overstrain the immune system of the incoming gilt. In addition, the gilts could them be carrying pathogens that are not present in the herd’s resident sows and therefore would become a source of infection. Clinically non-apparent diseases and microbe loads may frequently cause non-estrus or returning to heat.

Investigations in five farms

Our own investigations have aimed to analyze the possible relationship between the age of the gilts at introduction into the herd and their fertility at first farrowing.

The analysis was carried out with a total number of 2,269 gilts in five weaner producing herds ranging in size from 170 to 730 sows, from 2003 to 2007. Two breeds were included (Dalland and the German BHZP breed). The age at introduction varied between the units, therefore at first each farm was analyzed separately.

In a second step, the data were sorted according to breed (only Dalland gilts were included) and by the age at introduction, to obtain a largely homogeneous sample.

The parameters registered for each gilt were: age at introduction into the herd, at first insemination and at farrowing; number of inseminations leading to pregnancy; farrowing rate; litter size total and alive born piglets; breed; year and farm. The main parameters were litter size for total piglets born (because the number of liveborn piglets per litter can be influenced by the circumstances around farrowing), the farrowing rate and the piglet index.

The farrowing rate represents the number of gilts which farrowed related to the number inseminated. A farrowing rate of 85% means that, out of 100 gilts inseminated, 85 gave birth. The piglet index is defined as the number of total born piglets per 100 inseminated gilts. The index is calculated from farrowing rate multiplied by litter size (total born) and can range widely, for example from 700 to 1300 piglets born per 100 gilts or sows inseminated.

The index presents a very important parameter to assess the fertility of a herd. Only gilts which became pregnant after the first insemination were included in our own study whereas gilts that returned to heat were excluded from analysis. The characteristics of the five farms are summarized in Table 1.

Early introduction leads to an improved performance

In farm 1, a total of 1,137 gilts (Dalland breed) with a mean age of 118 days and a variation from 53 to 217 days at introduction were included. The gilts were classified into three categories concerning the age at introduction: up to 84 days, 85-160 days and over 160 days. The total born piglets for litters of gilts in the highest age category at introduction (> 160 days) was 11.74 on average. Therefore their result was 0.53 piglets per litter less than for group 2 (average 12.27) and 0.3 piglets/litter less than in group 1 which had the youngest gilts at introduction (Table 2). The oldest gilts (> 160 days) had a lower farrowing rate (76.8%) than the other two groups. Group 2, with the age range 85-160 days at introduction, gave the highest value (81.9%) as in Figure 1.

The piglet index (farrowing rate multiplied by total born piglets) of gilts inseminated successfully first time (so, without returning to heat) showed a difference of 58 to more than 100 piglets per 100 inseminated gilts in favour of the gilts introduced at a younger age (Table 2).

In farm 2, 174 BHZP gilts aged between 174-227 days at introduction were analyzed. They were divided into two classes: up to192 days and over 192 days. At 78.4%, the farrowing rate of the older gilts was 6.5 percentage points less than for the younger animals (84.9%). Taking into account the effect of the year (2003 to 2005) and a mean age at first insemination of 256 days, the younger gilts at integration had a higher litter size of 0.55 total born piglets as compared to older gilts (Table 2). The younger animals yielded over 100 piglets born per 100 inseminated gilts more than gilts introduced at a higher age.

Similar results were obtained in farm 3 with 473 Dalland gilts. Gilts introduced younger had a better fertility than their older companions. The gilts were classified into three categories of age at integration: under 150 days, 150-156 days and over 156 days. The highest farrowing rate was achieved by females with an age at introduction of 150 to 156 days (92.0%) followed by the younger pigs (88.4%) and by the older pigs with an age of more than 156 days (84.4%) (see Figure 1).

The youngest gilts at integration had the highest litter size (12.26 total born), whereas the oldest gilts had the lowest result in this parameter (11.10). They obtained nearly 150 total born piglets per 100 inseminated gilts less than the gilts of the other two categories.

In farm 4, the analysis used 215 gilts of Dalland breed with a wide range in age (56 to 290 days) at introduction. Because of this wide variability, three age groups were formed: up to 100 days, 100-265 days and over 265 days. The results were similar to those of farms 1 and 3. The highest farrowing rate was performed by the gilts of the medium age group (90.4%), the lowest by the female pigs which were introduced late (83.3%) as in Figure 1. The same tendency was found in litter size and in piglet index (Table 1).

The 270 Dalland gilts in farm 5 were divided in two groups concerning the age at integration: up to 177 days and over 177 days. Results in this farm agreed with those from the other four farms. The gilts with the lower age at introduction had a higher farrowing rate (84.3% versus 77.7%) and a higher litter size (11.33 total born versus 10.70) than the older gilts. This led to a difference in the piglet index of 124 total born piglets per 100 inseminated gilts. Both groups of gilts had the same age (248 days on average) at first insemination.

In the past, the weaner producers we investigated had introduced their gilts at around 180 days old. Summarizing the results of our own investigations, a positive effect of an earlier introduction on the reproductive performance was shown in all five farms. However, this impact was not significant because of a limited number of gilts.

Therefore, in a second step of analysis, the data from 1,819 Dalland gilts from four farms were merged. Only gilts with an age at introduction of less than 200 days from the years 2003 to 2006 were included. Those gilts were divided into two age categories: up to160 days and over 160 days at integration.

A significant influence of the age at introduction on the farrowing rate after the first successful insemination (without taking into account gilts returned to heat) was shown in the comparative homogeneous group regarding age, genotype and conditions of introduction. Younger gilts at introduction had a significantly higher farrowing rate (84.1%) than the older ones which averaged 180 days old at introduction, for which a farrowing rate of 80.2% is shown in Table 3. The age at integration also significantly influenced the total number of piglets born per litter.

Younger gilts had with a mean litter size of 11.64 total born, or 0.5 piglets more per litter than the 11.1 average of the older gilts. It must be emphasized that the age at first insemination in both groups was identical (249 days). The younger gilts thus had more time to adapt to the new microbial environment and reacted with a significantly better performance (an extra 89 total born piglets per 100 inseminated gilts).

If the group of the younger gilts was subdivided into two groups (up to 120 days and 121-160 days), it was shown that gilts with an age between 4 and 5.5 months at introduction achieved the best performance in the first litter. The farrowing rate was high (86.7%) and the litter size was good (12.26 piglets per litter). The piglet index was much higher in this group compared with the gilts of younger or older age at integration (Figure 2).

Possible reasons for the somewhat poorer performance of the youngest gilts (less than four months at integration) could be found in worse conditions for the young animals at the breeding herd, such as problems with space per pig and with feeding. The causes for these problems could not be detected exactly. These gilts had, however, a higher performance at first farrowing (measured by farrowing rate, litter size and thus piglet index) than the gilts introduced later meaning after 160 days old.

On balance therefore, considering the increasing differences that can be seen for microbiological situation between gilt multiplication units and weaner-producing herds and also bearing in mind the health problems caused by a too-fast introduction of gilts, a successful strategy might be to introduce the gilts earlier. The gilts then have more time to adapt to the new microbial environment.

As a result, a significant improvement in the fertility performance (on average + 4% in farrowing rate and + 0.5 total born piglets per litter) can be expected. In our own investigations, the gilts with an age of approximately five months at introduction had the best performance at first farrowing.