Breeding programs toward hyperprolific sows have resulted in sows that produce a surplus of piglets compared to the number of functional teats (1). The Swedish national average of live-born pigs per litter has risen from 13.1 to 14.0 from 2011 to 2016 (2), and the average litter size is expected to rise even further. The intention with the increased litter size is a more efficient and profitable pig production by a higher number of piglets that can be weaned and later slaughtered, without increasing the number of sows. However, in large litters more piglets will have to share the resources (e.g., milk), and a sow usually has 14 teats and can thus feed up to 14 piglets. In today’s production, a sow can give birth to more than 20 live-born piglets. If no action is taken, not all of these piglets can be expected to survive.

Most often farrowing occurs batch wise, i.e., a whole group of sows farrows within a few couple of days. This makes it possible to move piglets from large litters to sows with smaller litters, known as “litter equalization” (1). In this manner, the survival rates might increase (3). If the number of functional teats in the farrowing group is not sufficient, surplus piglets can be gathered and placed with a nurse sow. A nurse sow is a lactating sow from another farrowing group that have just weaned her own litter (4). The nurse sow is moved into the group of fresh sows and will continue to be suckled by a new litter of foster piglets. Nurse sows can be used in one-step or two-step systems (1). A one-step system means that a sow, immediately after weaning her own piglets, is moved to a foster litter with newborn “surplus piglets”. The one-step nurse sow will have a total lactation period of at least 8 weeks (28 + 28 days) under Swedish conditions. Another way to use nurse sows is to do it in two steps. In this way two sows are needed. Sow 1 nurses her own litter for 4–8 days and then the whole litter is moved to Sow 2, which has just weaned her own litter. Sow 1 receives newborn surplus piglets and nurses them for at least 28 days if in Sweden. Sow 1 will have a total lactation period of approximately 5 weeks (4–8 + 28 days) which is comparable to the average lactation period in Sweden [33 days (2)]. Sow 2 will have a prolonged lactation period of approximately 3 weeks depending on how old the foster litter is when it is moved. If the foster litter is 4 days when transferred, the nurse sow needs to have them for at least 24 days before they can be weaned. This gives a total minimum lactation period of 7.4 weeks (28 + 24 days) for Sow 2 if she weaned her own litter after 28 days. The overall lactation period for the nurse sows is shorter in the two-step nurse system. Older foster piglets are also more easily accepted compared to newborn by sows at weaning (5). The two-step nurse system is known to be more commonly used in Denmark where nurse sows are used to a wide extent (6).

The Swedish legislation has more stringent rules regarding the keeping of sows and piglets during suckling than the European Council Directive (7). Sows in Sweden should be loose housed at both farrowing and suckling (SJVFS 2010:15), which is not a requirement in the rest of the EU where the sows can be kept in crates during this period (7). Furthermore, in Sweden, piglets have to be at least 28 days at weaning. This is generally valid throughout the EU, but an exception to the regulations makes it possible to wean piglets already at 21 days, which is widely practiced in nurse sow systems in order to limit the overall lactation period for the nurse sow (1). This means that nurse sows in Sweden will have longer lactation periods compared to nurse sows in other countries, and this may have an impact on their welfare, but the welfare aspects of nurse sow systems in Sweden have not been addressed so far.

There are also no published studies on the economic aspects of using nurse sows under Swedish production conditions. The objectives of this study were therefore to: (1) discuss possible animal welfare consequences associated with using nurse sows under Swedish conditions, with a standpoint from available published literature and expert opinions and (2) evaluate the effects of using nurse sows on the contribution margin of piglet production in Sweden.

As there is a paucity of literature on nurse sows, and no published studies from Sweden, an expert group meeting was organized to gather information on the nurse sow system and its effects on animal welfare and production parameters. This information was a necessary first step to build an economic model as results and figures from available literature could not be directly transferred into the economic models as the Swedish production conditions differed in many ways. The expert group meeting was arranged in Uppsala, Sweden, in August 2016 and included persons from the industry working as herd health veterinarians, production advisors, and researchers. The meeting started with a brief presentation of the results from a literature review consisting of 36 references including published papers, scientific reports, and a bachelor thesis. Different animal welfare aspects of using nurse sows on herd level as well as individual level, both from the sow and piglet perspective, were then discussed according to a structured protocol. In the next step, the economic parameters included in a standard template (8) were discussed one by one. Finally, the group agreed collectively on which input variables to include in the economic model for this study as well as their effects. The expert group meeting lasted for 3 h.

To estimate the economic aspects of using nurse sows, a partial budget (contribution-margin)-based stochastic farm-level model was developed in Microsoft Excel 2013 (Microsoft Corp., Redmond, WA, USA). In this way, we could isolate the effects on the contribution margin of using nurse sows compared to a conventional system, by only focusing on the economic variables (revenue and costs) likely to depend on the system. The model was based on a standard template (8), but modified according to experts’ opinions to include most variables affected by using nurse sows. Input variables were based on Swedish pig production data from 2015 and 2016 (9) and the results from the expert group meeting. The input variables are listed in Tables 1 and 2.

The model scenario was based on a farm with three farrowing stalls and farrowings fortnightly. The conventional system had 590 sows/year, and the nurse sow scenario had 567 sows/year. Each farrowing group was kept in stalls with 50 farrowing pens. In the conventional system 50 pregnant sows were housed in the pens at start, while in the nurse sow scenario 48 pregnant sows were housed and the remaining 2 pens were used for surplus piglets, and two-step nurse sows (from another farrowing group) were later moved to these pens. The sow groups were moved to the farrowing unit 4 days before expected farrowing and kept in that unit until all piglets had reached 28 days of age. In this way, the sow group stayed in the farrowing unit for a 5-week period.

The basic model was deterministic, however, some key variables were modeled stochastically (Tables 1 and 2). A stochastic model takes the parameter variation into calculation to generate results with a distribution, representing uncertainty in results (12). The stochastic elements of the model were handled with @RISK 7.5 (Palisade Corp., Ithaca, NY, USA), an Excel add in, which performs risk analysis using Monte Carlo simulations. In each simulation, 5,000 iterations and Latin Hypercube sampling were used with a static seed of 31,517 to ensure that all simulations provided repeatable results.

The output variables affecting the contribution margin in the partial budgeting were “Revenue from sold piglets,” “Feed costs”, and “Semen costs.” “Revenue from sold piglets” was calculated by using the number of weaned piglets per farrowing group (the number of live-born piglets per farrowing group × piglet mortality rate) and the price at sale [580 Swedish krona (SEK) per 30 kg batch-pig weight with an addition of 6.5 SEK per kg for batches with average weights > 30.1 kg/pig]. “Feed costs” consisted of the number of feed weeks × feed consumption × feed price (for all categories, i.e., sows in dry period, lactating sows, and growing piglets). “Semen cost” was calculated as the number of sows × cost for semen. A correlation between number of live-born piglets and piglet mortality rate for the conventional system and the nurse sow system was set to 0.8 and 0.5, respectively. The impact on contribution margin (revenues minus costs) of using nurse sows was compared to a conventional situation and was calculated at batch level for one farrowing group (50 pens) from insemination (included one dry and one lactation period). The full economic model is shown in Table S1 in Supplementary Material.

Further contact was made with the expert group to sort out upcoming queries and to validate the model. Finally, a sensitivity analysis was used to evaluate input variables with strong impact on the model outcome. This was conducted using the sensitivity analysis function of @Risk. Regression tornado diagrams, in which @Risk runs a multiple regression analysis for each iteration with the outcome of interest and the simulated (standardized) values of the stochastic variables as independent variables, were carried out. This analysis shows the mean of the 10% lowest and highest simulated values (Tornado graph “change in output mean”) and the change in the outcome variable when the independent variables increase by 1 SD with all other variables being constant (Tornado graph “regression mapped values”).

The objectives of this study were to discuss the animal welfare aspects and to explore the economic aspects of using nurse sows in Swedish pig production. The results suggest that there are animal welfare concerns both for sows and piglets that need to be considered and also that using nurse sows can provide a slightly better economic performance than conventional practice on Swedish farms. However, it should be noted that in the economic model, the definition of contribution margin did not include cost items related to salary and veterinary costs, as these variables are considered relatively constant in the short run. Veterinary costs at pig farms in Sweden are usually constant because almost all farms have a fixed number of scheduled veterinary visits per year and need then not be included in a contribution margin analysis (which is conducted within the limits of given fixed resources). Salary was not added to the model as the amount of person-hours needed in the analysis was too difficult to estimate, because only a bachelor thesis on labor use in nurse sow systems in Sweden has been done. Moreover, labor is a production factor that in reality can be considered fixed in the short run, as it usually takes time to recruit new employees and it could be difficult finding persons interested in part-time employment, and should then by definition not be included in the calculation of contribution margin. Furthermore, the animal welfare implications are not included in the economic model, even though there are several important animal welfare aspects regarding both sows and piglets. Direct economic effects of such aspects are, however, difficult to assess and research into ways to transform ethical values into monetary terms is strongly advocated.

The general goal with nurse sows is to increase the piglet survival rate (from birth to weaning). In our scenario, the mortality rate was set to be higher in the conventional system, an input which was based on the expert solicitation exercise because no studies were available. The sensitivity analysis showed that this variable had the largest influence on the outcome, and it would therefore be important to estimate piglet mortality in nurse sow systems, especially under Swedish conditions, to verify our results. The second most influential variable was number of live-born piglets, but the input value of variable was based on large number of production records, and the sensitivity analysis thus shows the effect of an inherent variability.

It is important to remember that there is a large variability in most input variables on commercial farms, and the results from this study only give an indication that the nurse sow system can be profitable under average Swedish production conditions. The beneficial potential of using nurse sows will highly depend on the conditions and management routines at specific farms. The economic model in this study included only a few number of variables. Parameters not included in this economic model, e.g., labor, may influence the results, and a cautious interpretation of the results is recommended. There could also be other important welfare aspects (e.g., behavioral and physiological parameters), additional to the ones discussed in this paper, which need to be considered in future studies.

Using nurse sows is one way to reduce piglet mortality rates from birth to weaning. The Swedish national average of live-born piglets is 14.0 per litter. Breeding toward larger litter sizes will result in lower average birth weights for piglets, but also to an increased risk of non-sufficient colostrum intake because of the competition for functioning teats. This study explored the animal welfare and economic consequences of using nurse sows under Swedish production conditions to overcome some of these problems. In the nurse sow system (assuming a fixed number of farrowing pens), there will be less sow-years compared to the conventional situation. The partial budget analysis showed a higher contribution margin for the nurse sow system. Animal welfare aspects were, however, not included in the economic model, and due to the limited number of input variables the results should be interpreted with caution. There are important animal welfare concerns that need to be studied further, especially under Swedish production conditions.

All authors contributed to the design of the work. KA, HH, and RW participated in the expert group meeting. KA, HH, and UE performed the analysis. KA drafted the manuscript, and all the authors read and approved the final manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.