Forage particle size, rather than dietary iNDF concentration, affects chewing activity and lactation performance in lactating dairy goats.

This study investigated the independent and interactive effects of forage particle size (PS; short: 2 mm vs. long: 20 mm) and dietary indigestible neutral detergent fiber (iNDF; low vs. high) on intake, chewing, rumen fermentation, and lactation performance in sixty multiparous Mohabadi dairy goats (2 × 2 factorial). Long PS reduced dry matter intake (1334 vs. 1472 g/d; P < 0.01) and milk yield (1051 vs. 1405 g/d; P = 0.036), increased milk fat (3.99%vs. 3.68%; P < 0.01), and lowered feed efficiency (0.850 vs. 0.983; P < 0.01). Eating time (199 vs. 86 min/d), ruminating time (452 vs. 349 min/d), and total chewing time (691 vs. 407 min/d) were markedly increased by long PS (P < 0.01). Dietary iNDF did not significantly affect DMI, milk yield, or milk fat (P ≥ 0.18), but low iNDF improved NDF digestibility (51.6%vs. 47.8%; P = 0.05) and reduced blood urea nitrogen (9.55 vs. 11.91 mg/dL; P = 0.02). Rumen pH was stable (6.47-6.72), and most fermentation variables did not differ; only a significant main effect of PS on acetate (P = 0.02) and a main effect of PS on valerate (P = 0.02) were detected. Significant PS × iNDF interactions for some intake and sorting variables indicated that iNDF partially modulated PS effects. Overall, forage PS exerted a far greater influence than iNDF on intake, chewing, and milk production, with only minor rumen fermentation changes, underscoring the dominant role of physically effective fiber.