When breaking down the different phases that a sow has to go through in its productive cycle, one of the most important in terms of farm profitability is the WSI, which by definition is the period that occurs since we wean a lactating sow until its subsequent service. Although in a practical way this value gives us a lot of information on how the breeding herd is being managed (lactation, nutrition, heat detection, etc.), the most appropriate in order to carry out a more exhaustive analysis is to separate this indicator into Interval weaning to first service and weaning to conception interval (on which day conception actually occurs). In this way we provide information on how the insemination procedure is being carried out.
Even though its duration is relatively short (5.91 WSI days and 8.89 WCI days on average in Spain, 2020), its effect is decisive on the profitability of the farm since its increase, linked to the number of repetitions and the rate of anestrus generate an increase in non-productive days (NPD), that is, days in which sows are neither pregnant nor lactating, but still incur costs (approximately € 2.5 per day in Spain, 2020). In case of failure in the mating, we will have to wait 21 days, in which in addition to the NPD costs we will have to add the opportunity costs (18 kg piglet: € 94; 110 kg pig: € 225 in Spain, 2021).
Another technical-economic indicator in which the WSI contributes is the number of farrowings per sow and year (2.31 Spain, 2020), also being the component in which we can influence more directly, since the others will depend on the genetics (days of gestation) and the management of the batches and available facilities (day of lactation) factors that are highly adjusted and that are difficult to modify in the short term.
Different studies also indicate benefits in future productivity depending on when the service is accomplished. Improvements in the prolificacy rate (birth rate and total births) were observed in services do it on day 4 vs. day 5, regardless of the parity number of the sow (PigChamp, 2017) improvement that was also observed in the studies carried out by Wilson and Dewey (1993) and Ketchem (2017), this interval being also repeatable in their subsequent cycles (Yatabe 2019). The time that will elapse until the start of heat after weaning will also determine the duration of the heat itself. This will determine the choice between different service protocols with different results in the reproductive rates (Tarocco, 2001).
In order to reduce the duration of this interval, the main action points and how they influence its improvement are described:
The body condition of a sow at the end of lactation is a reflection of how nutrition was carried out in this phase, being also the main determinant of the duration of the WSI (Estienne 1999). Koketsu (1997) estimated an increase of 0.12 days for every 1 kg less consumed in lactation. Optimizing consumption in lactation is the foundation that will allow us to have a productive sow. At post-weaning, sows that consumed insulinogenic diets (rich in starch and sugars) showed better performance due to their beneficial effect on the release of hormones involved in reproduction (Van de Brand, 2000; Silva, 2017). Supplementation with different feeds such as spray-dried plasma (Crenshew, 2007), biotin (Koketsu, 1997) and betaine (Ramis, 2011) also improved reproductive rates. The flushing effect (diets high in energy and high intake) is a beneficial strategy, only in those animals in which good feeding management was not carried out in lactation and in primiparous, as suggested by the study carried out by Almeida (2020), which it is recommended to do an individualization of the feeding in the WSI.
: It is important to start to check the same day of weaning, not only to detect those sows that can begin to cycle already in lactation but also to heat stimulate (Ketchem, 2017), paying special attention to gilts that can show a less evident and / or shorter heat. It is also important to know that if they show heat before day 4, it will be longer (Kemp, 1996) so it is recommended to wait 24 hours to perform the first insemination.
It will be very valuable to control stressors, since due to hormonal inhibition mechanisms, they will not allow to exhibit signs of heat in addition to the decrease of feed intake, a fundamental factor in the success of the phase, as mentioned. Among the most common causes of stress are heat (the cause of summer seasonal anestrus), overcrowding, noxious gases, pain, and annoying noise.
Part of the energy consumed in first farrowing sows is destined for their own growth, leaving relatively less energy for reproduction, so that a high percentage of first farrowing sow will cause longer WSI. The number of primiparous should not be more than 20% of the total sows.
The use of hormones such as chorionic gonadotropin and pregnant mare's serum gonadotropin have shown a significant improvement in heat induction, mainly in anoestric sows (Falceto, 2014).
Luminosity is very important in the pigs reproduction, a minimum of 300 lux is recommended at the level of the sow's eyes for 16 hours a day (Gadd, 2016)
The use of ultrasound is a very helpful tool in the reproductive management of the sow. It allows us to evaluate the health of the reproductive tract at weaning, monitoring ovarian activity and estimating the time of ovulation, thus optimizing inseminating doses and culling sows with pathologies.