Replacing corn with full-fat corn germ in a basal diet containing cactus (Opuntia stricta) cladodes and sugarcane as forage sources induces milk fat depression associated with the trans-10 shift in dairy cows

Abstract

Recent evidence indicates that associating cactus Opuntia cladodes with plant oils represents a promising strategy to increase the nutritional value of milk fat. This study aimed to investigate the effects of full-fat corn germ (FFCG), as an alternative to plant oils, on production variables, blood metabolites, and milk fatty acid (FA) composition of dairy cows fed a basal diet containing cactus (Opuntia stricta) cladodes and sugarcane as forages. Ten multiparous Holstein cows with 90 ± 10 d of lactation and yielding 24.2 ± 3.5 kg milk/d were assigned to dietary treatments consisting of 0, 4 (low level; LL), 8 (moderate level; ML), 12 (high level; HL), and 16% (very high level; VHL) of FFCG (DM basis) in a replicated 5 × 5 Latin square design with 21-d periods. DM intake decreased linearly from 18.9 to 16.4 kg/d, whereas a quadratic response was observed for milk yield, which increased 1.7 kg/d from control to ML, and decreased to a similar extent as FFCG further increased in the diet, resulting in a quadratic increase in gross feed efficiency (GFE). Milk protein and lactose contents were unaltered, but milk fat content gradually dropped from 3.49% to 2.23%, which was accompanied by a progressive decrease in milk fat yield and fat-corrected milk yield. Serum cholesterol levels markedly increased from 756 to 1480 mg/L, but other blood metabolites were unaltered. Except for trans-16:1 isomers, proportions of short and medium-chain FA in milk fat decreased in a linear or quadratic fashion as FFCG increased in the diet, whereas the opposite was observed for 18:0 and of 18:1/18:2 isomers, including those with conjugated double bonds. In particular, a trans-11 to trans-10 shift occurred with moderate to higher FFCG levels, resulting in a quadratic change in trans-11 18:1 and cis-9, trans-11 CLA, and a linear increase in trans-10 18:1, trans-10, cis-12 CLA and trans-9, cis-11 CLA. The n-6:n-3 ratio in milk nearly doubled with increasing FFCG, mostly due to a pronounced increase in 18:2 n-6. Proportions of odd-linear and branched-chain FA in milk fat decreased in a quadratic and linear fashion, respectively, with increasing FFCG. These results indicate that FFCG can be an effective source of PUFA for increasing rumen outflow of biohydrogenation intermediates in cows fed cactus Opuntia cladodes as a partial substitute for sugarcane. The marked milk fat depression induced by FFCG suggests that the basal diet was unable to maintain a healthy rumen environment.