Birth Type Lastingly Programs Secondary Hair Follicle Population and Cashmere Production in Cashmere Goats Despite Full Somatic Catch-Up Growth

The objective of this study was to investigate the effects of birth type on growth performance, cashmere production traits, and hair follicle populations in cashmere goats. A total of 74 newborn kids (single, n = 38; twin, n = 36), all sired by a single Inner Mongolian Cashmere buck, were used in this study. Body weights were recorded at birth, weaning, and yearling age to calculate pre- and post-weaning average daily gain (ADG). At yearling age, cashmere samples and skin biopsies were collected for measurement of cashmere yield, fiber diameter, staple length, and primary and secondary hair follicle population parameters. Results showed that single-born kids exhibited significantly greater birth weight by 18.3% (2.85 ± 0.06 kg vs. 2.41 ± 0.06 kg), weaning weight by 10.3% (20.17 ± 0.32 kg vs. 18.28 ± 0.17 kg), pre-weaning weight gain by 9.2% (17.31 ± 0.31 kg vs. 15.85 ± 0.20 kg), and pre-weaning ADG by 14.5% (146.5 ± 3.4 g vs. 127.9 ± 1.6 g) than twin-born kids (p < 0.05). Conversely, twin-born kids surpassed single-born kids in post-weaning weight gain by 14.4% (14.60 ± 0.48 kg vs. 12.76 ± 0.48 kg) and post-weaning ADG by 16.8% (60.5 ± 1.9 g vs. 51.8 ± 1.9 g), indicative of clear compensatory growth, resulting in no significant difference in yearling weight (32.90 ± 0.51 kg). Birth type significantly affected yearling cashmere production: single-born kids produced 10.1% more greasy cashmere (810 ± 22 g vs. 736 ± 28 g; p = 0.046) and exhibited a 0.45 μm finer mean fiber diameter (14.34 ± 0.10 μm vs. 14.79 ± 0.11 μm; p = 0.003), whereas staple length did not differ between groups (9.70 ± 0.12 cm). Consistent with the fiber trait results, no differences were detected in any primary hair follicle population parameters. By contrast, all secondary hair follicle traits were significantly higher in single-born goats, with secondary follicle density (SFD) being 10.6% greater (36.6 ± 1.0 vs. 33.1 ± 0.8; p = 0.010), secondary follicle density index 11.7% greater (363.9 ± 9.4 vs. 325.7 ± 7.6; p = 0.003), secondary follicle number 12.4% greater (33.46 ± 0.87 vs. 29.78 ± 0.70; p = 0.002), and the S/P ratio 8.9% greater (12.11 ± 0.36 vs. 11.12 ± 0.29; p = 0.041). All secondary follicle population parameters were negatively correlated with fiber diameter, among which SFD showed the strongest association (r = −0.70, p < 0.001). These findings indicate that birth type is associated with lasting differences in secondary hair follicle endowment and cashmere production, which persist at least through yearling age despite full compensatory body growth in twin-born kids. The observed association is consistent with the hypothesis that the pre-weaning period constitutes a critical window for follicle maturation—a window during which secondary follicle development is highly sensitive to nutritional status during gestation and early postnatal life. This suggests that targeted nutritional management of twin-bearing does during late gestation and early lactation may represent a promising strategy to partially mitigate the disadvantage in cashmere production potential imposed by twinning. However, direct causal evidence for both the developmental programming hypothesis and the efficacy of such nutritional interventions requires further experimental validation, including longer-term studies tracking animals across multiple production cycles and controlled nutritional trials.