Increased growth rate post-weaning affects mammary gene expression of two-year-old ewes during their second pregnancy and lactation

This study examined the long-term effects of increased growth rates between three and seven months of age on gene expression in the mammary gland of nondairy ewes during their second lactation. A total of 19 twin-bearing, two-year-old ewes that either had an increased growth rate between three and seven months of age (heavy; n = 9; 153 ± 2.2 g/d and 47.9 ± 0.38 kg at breeding) or did not have an increased growth rate (control; n = 10; 127 ± 1.9 g/d and 44.9 ± 0.49 kg at breeding) were selected. Mammary gland biopsies were collected at day 128 of pregnancy and day 30 of lactation to examine the expression of 37 genes involved in mammary cell development and milk fat metabolism using NanoString nCounter technology. Ewes in the heavy group tended to exhibit lower expression (P < 0.05) of acyl-CoA synthetase short-chain family member 1 (ACSS1), a gene critical for mitochondrial acetyl-CoA synthesis, energy production, and milk fat synthesis, and a trend toward (P = 0.11) lower expression of signal transducer and activator of transcription 5A (STAT5A), a regulator of mammary epithelial cell differentiation and survival. These lower expressions suggest potential carry-over effects of increased growth rate between three and seven months of age. However, no phenotypic differences were observed in lamb growth or live weight, and no differences were detected in the expression of downstream target genes or modulators of these pathways, suggesting limited functional impact on mammary gland development and lamb performance outcomes. Further investigations, including a functional assessment of lactation and use of comprehensive transcriptomic analyses, would be needed to understand the effects of increased growth rates between three and seven months of age on ewe mammary function and milk composition.