Low n-3/High n-6 Flaxseed and Hermetia illucens Larvae Oils as Feed Materials: Effects on In Vitro Rumen Fermentation, Maternal Metabolism, Antioxidant Status and Colostrum in Prolific Ewes

Black soldier fly larvae (BSFL; Hermetia illucens) oil and flaxseed oil provide contrasting fatty acid profiles and may serve as alternative lipid feed materials in ruminant nutrition. This study assessed low-α-linolenic/high-linoleic flaxseed oil, BSFL oil, and their blends using an in vitro rumen fermentation screening with pooled bovine rumen inoculum and a randomized prepartum feeding trial in 32 twin- or triplet-bearing Olkuska ewes. In vitro, the single oils and a 1:1 blend were evaluated at two inclusion levels; total VFA concentration was unchanged, whereas propionate proportion, the acetate-to-propionate ratio, methane, ammonia, and incubation-fluid fatty acid profiles showed treatment-dependent responses. In vivo, an unsupplemented control was compared with flaxseed oil, BSFL oil, and a 2:1 blend supplied at 30 g/day for 21 days before expected parturition. Colostrum fat concentration differed among treatments (p = 0.036); the 2:1 blend group had a higher value than the control group (11.29% vs. 8.26%; Dunnett-adjusted p < 0.05), while major fatty acid classes did not differ. Supplementation was also associated with differences in selected minor colostrum fatty acids and several serum metabolic variables. Colostrum yield was not measured; therefore, this finding reflects fat concentration rather than total colostrum fat output. Serum total antioxidant status (TAS) was unaffected, while a treatment-by-time interaction was observed for γ-glutamyl transferase. The tested plant- and insect-derived oils therefore modified ruminal lipid transformations and selected maternal and colostrum traits without a detectable change in TAS.