Evaluation of the electrolyte status in hyperprolific sows on the farrowing process under different housing conditions

The reproductive performance of sows worldwide is of high standards. Key points for the profitability of pig breeding herds are the increase in litter size and the shorter reproductive cycle obtained by reducing the weaning to oestrus interval [1]. As a result, litter size has doubled from 10 to 20 piglets and a production up to 40 piglets weaned per sow and year seems likely [2,3].

However, it is questionable whether this increase in performance is associated with side effects relevant to the farrowing process, piglet survival and lactation. Most of the relevant literature from the past few years deals primarily with the piglet mortality [[4], [5], [6], [7], [8], [9]]. Depending on the type of husbandry, a mortality rate of between 12 and 25% must be expected during the lactation period [4,[10], [11], [12]]. Besides piglet crushing as the main housing factor, early losses within the first days of life are associated with small size at birth (<1000 g) or with adaptation problems [4,7,[13], [14], [15]].

In addition, the farrowing process in modern pig breeds is of increasing scientific interest. In some more recent publications, this is analyzed in more detail from an ethological and/or physiological point of view [5,8,10,[16], [17], [18], [19], [20]]. Other studies deal with the endocrine control of farrowing [[21], [22], [23], [24], [25]].

So far electrolyte-associated aspects of the peripartal period have received less attention. Nevertheless, the electrolyte status in this stage of the reproductive cycle is of particular importance. Macro elements such as calcium, magnesium and phosphorus are essential for the farrowing process and the onset of lactation [[26], [27], [28], [29], [30]]. Regardless of the intravascular status of relevant hormones for farrowing, insufficient concentrations of these elements in the myometrial tissue can lead to parturition problems [[31], [32], [33]]. Only if all components involved work synergistically progressive contractions during the expulsion stage can be assured.

All electrolytes must be present in sufficient intravascular concentrations. This is the only way to cover the increased demand of them during farrowing: Calcium is the most important second messenger in the biochemistry of myometrial contraction [34]. The calcium concentrations in the extracellular space and the intracytoplasmic reserves serve as stores [35,36]. The calcium present intracellularly enables the actomyosin reaction, the biochemical basis of uterine contractions. Due to depolarization of the cell membrane calcium concentration in the cell raises. This increase produces Calcium-Calmodulin and activates myosin light chain kinase [37]. The phosphorylation of myosin leads to a contraction. A Mg-dependent adenosine triphosphatase (ATPase) hydrolyzes adenosine triphosphate (ATP) into adenosine diphosphate (ADP) and phosphate, and a new contraction cycle begins as long as enough calcium and ATP is present in the smooth muscle cells [35]. An intact extra- and intracellular electrolyte balance is therefore necessary during farrowing [37]. Older studies addressing these issues mainly included sows with a small to medium litter size [[31], [32], [33],38,39]. It is also noticeable that in these publications, blood samples were taken randomly during farrowing [27,31,40,41].

The lactation period also attracted more attention [26,29,39,40,42,43]. It is at least questionable whether a transfer of these results to modern sows, especially for parturition, is possible. The increase in the number of fetuses and the associated chronic endogenous burden on the electrolyte balance could have a different effect on hyperprolific sows than on breeds with small to medium litter sizes. It is possible that this cumulative stress on the maternal organism during the end of gestation, farrowing and early lactation carries the risk of electrolyte imbalances.

The aim of this study was to evaluate the electrolyte ratios in hyperprolific sows in the peripartal period, especially during parturition. A further objective was to determine whether the type of housing, as an additional factor relevant to farrowing and animal welfare, influences the electrolyte status.

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