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

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.