Dietary net energy regulates puberty onset and estrus synchrony of replacement gilts via remodeling gut microbiota and serum metabolic profiling

Technical Summary: Dietary Net Energy Regulates Puberty Onset and Estrus Synchrony of Replacement Gilts via Remodeling Gut Microbiota and Serum Metabolic Profiling

Problem Statement
In modern intensive swine production, the involuntary culling of replacement gilts due to delayed puberty, persistent anestrus, and unsynchronized sexual maturation represents a primary constraint on herd longevity and economic efficiency. While nutritional strategies are known to influence somatic growth and reproductive tract development, there is a lack of systematic dose-response trials defining optimal dietary nutrient specifications to support targeted puberty timing and ideal first-mating body status. Furthermore, the intrinsic mechanisms linking dietary energy supply to reproductive development, specifically through the lens of gut microbiota and host metabolic profiling, remain unclear. Current feeding protocols often fail to match the metabolic phenotype of genetically improved modern gilts, creating a bottleneck in production efficiency.

Methodology
The study employed a large-scale, phase-specific feeding trial involving 366 Danish Landrace × Large White crossbred replacement gilts. The experimental design utilized five graded Net Energy (NE) treatments (T1–T5) across two rearing phases:

• Phase 1 (120–156 days): NE levels were set at 2,090, 2,180, 2,270, 2,360, and 2,450 kcal/kg.
• Phase 2 (157 days to pre-breeding): NE levels were adjusted to 2,040, 2,130, 2,220, 2,310, and 2,400 kcal/kg.

All other nutrient levels (e.g., crude protein, SID lysine) were standardized across treatments. Growth performance, puberty parameters (age, body weight, backfat thickness), estrus rates, and reproductive performance across the first two parities were recorded.

To elucidate the underlying mechanisms, the study selected the low-NE (T2) and moderate-NE (T4) groups for multi-omics integration:

1. Non-targeted Metabolomics: Serum samples were analyzed via LC-MS to identify differential metabolites.
2. Gut Metagenomics: Fecal samples were subjected to high-throughput sequencing to analyze microbial community structure, diversity, and functional pathways (KEGG).
3. Correlation Analysis: Spearman rank correlation was used to construct a microbe-metabolite interaction network.
4. Hormonal Profiling: Dynamic sampling of reproductive hormones (LH, FSH, E2, etc.) was conducted on day 17 of the first estrous cycle.

Key Results

• Growth Performance: Gilts fed the moderate NE diet (T4: 2,360 kcal/kg in Phase 1; 2,310 kcal/kg in Phase 2) exhibited the highest average daily gain (ADG) and best feed conversion efficiency (F:G). The highest NE group (T5) did not further improve growth, suggesting a threshold effect.
• Puberty Onset and Synchrony: T4 and T5 gilts reached puberty 6–8 days earlier than low-NE groups (T1–T3). T4 demonstrated the highest estrus rate before 200 days of age (81.94%). While body weight at puberty was unaffected, T4 gilts achieved a backfat thickness at first mating (14.52 mm) that fell within the optimal reproductive range (14–15 mm), superior to T1 and T2.
• Hormonal Dynamics: Although basal estradiol (E2) levels were lower in T4, dynamic sampling revealed significantly higher serum luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels in T4 compared to T2, indicating enhanced pituitary secretory activity and follicular maturation.
• Metabolomics: 107 differential serum metabolites were identified between T2 and T4, primarily involving glycerophospholipids, triglycerides, organic acids, and small peptides. T4 was characterized by a metabolic profile favoring energy mobilization and lipid signaling.
• Metagenomics: While alpha diversity remained unchanged, T4 exhibited a distinct microbial structure enriched with Streptococcus gallolyticus and Streptococcus equinus. Functional analysis showed T4 microbiota had enhanced pathways for carbohydrate catabolism, ABC transporters, and the biosynthesis of amino acids and vitamins.
• Integrated Mechanism: Correlation analysis revealed that T4-enriched Streptococcus species were positively correlated with upregulated organic acids and phospholipids, suggesting a regulatory cascade where gut microbes modulate host metabolism to activate the hypothalamic–pituitary–ovarian (HPO) axis.
• Reproductive Performance: Crucially, dietary NE levels did not significantly affect total number born, number born alive, farrowing rates, or retention rates across the first two parities.

Significance and Claims
The paper claims to establish a practical, phase-adjusted nutritional framework for modern replacement gilt management. The primary contribution is the identification of specific NE levels (2,360 kcal/kg and 2,310 kcal/kg for the respective phases) that optimize growth and accelerate puberty onset without compromising early-parity reproductive performance.

The study provides multi-omics mechanistic evidence that dietary NE regulates gilt reproductive development via a "gut microbiota–host metabolism–HPO axis" model. Specifically, it posits that moderate NE levels remodel the gut microbiota (enriching Streptococcus spp.), which in turn reprograms serum metabolism (lipid and organic acid pathways) to synchronize puberty and enhance estrus rates. The authors conclude that this approach resolves the mismatch between traditional feeding protocols and the biological potential of genetically improved gilts, offering a strategy to reduce involuntary culling by ensuring a higher entry rate of well-developed, cyclic gilts into the breeding herd. The paper notes that while early reproductive metrics were stable, long-term trials covering more parities are necessary to fully evaluate lifetime productivity effects.