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Background
The modern breeding of terminal line swine, selected for enhanced reproductive traits [1] such as litter size, exacerbates the challenge for the primiparous sows to obtain sufficient nutrients during lactation, creating excessive weight loss and bone density reduction, leading to failure to rebreed for subsequent reproductive cycles [2]. These challenges are found in multiparous sows to a lesser extent as parity progresses. Therefore, it is often observed that reproductive performance continuously increases and peaks between parities 3 and 4 [3]. However, sows that fail to rebreed and are subsequently removed from the production cycle can cause significant economic losses in the swine industry.
Parity, which refers to the number of litters a sow has given birth to, is a crucial metric of sow reproductivity in breeding operations [4]. Reproduction traits, including litter size, piglet birth weight, and progeny weight gain during lactation, are substantially affected by parity due to the impact of young sows’ still-developing physiological functions and requirements on their milk yield and milk composition [5]. In addition, parity also affects sow growth, feed efficiency [6], and birth intervals [7]. Recycling of sows through multiple parities and promoting reproductive performance increases the efficiency and productivity of the swine industry. Hence, screening for gilts with increased reproductive longevity potential in early life is of great research interest.
The selection for the breeding of modern swine involves a comprehensive approach that combines various techniques [8], such as phenotypic selection [9], crossbreeding [10], genomic selection [11], and artificial insemination [12]. It has been widely reported that the microbiota is associated with host physiological function [13, 14–15], pregnancy [16, 17], and reproductive performance [18]. Interestingly, recent research has indicated that the gut and vaginal microbiota could affect estrus return in post-weaning sows through its effect on sex hormones [19, 20]. These studies suggest that the microbiota serve as a potential method for screening for sows with increased reproductive efficiency traits. Importantly, microbes dynamically change within different growth or physiological periods. Thus, understanding how the microbiota varies within specific stages throughout the swine life cycle helps develop targeted strategies to improve the health and productivity of pigs. It has been illustrated that the composition of the swine fecal microbiome is altered between the pregnancy and lactation stages [17]....