Grazing allowance after the morning or afternoon milking for lactating cows fed a total mixed ration (TMR) enhances trans11-18:1 and cis9,trans11-18:2 (rumenic acid) in milk fat to different extents

During a 6-week period, three groups of 10 cows were fed a total mixed ration (TMR) only or fed the TMR plus access to mixed grass/clover pastures for 8 h after the morning (PAM) or afternoon (PPM) milking to determine the influence of voluntary pasture intake on profiles of biohydrogenation intermediates in milk fat. Cows in PAM had access to paddocks (0.2 ha per day) from 03:00 to 11:00 h, whereas cows in PPM were allowed to graze from 15:00 to 22:00 h. Estimated intake of dry matter (DM) from the TMR by cows within groups was 25, 20, or 16 kg per day, respectively, for TMR, PAM, or PPM. Milk yield was not affected by treatments averaging 31 kg per day, and concentrations and yields of milk components were also unaffected. Milk fat concentration of trans10-18:1 was higher for cows fed TMR compared with PAM or PPM. In contrast, concentrations of trans11-18:1 in milk fat from cows fed PAM (20 mg/g total fatty acids) or PPM (24 mg/g) was higher compared with those fed the TMR (14 mg/g). Cows fed PAM or PPM also had greater concentrations of cis9,trans11-18:2 (9 or 10 mg/g) compared with those fed TMR (6 mg/g). Thus, daily yields of trans11-18:1 averaged 17, 20, and 13 g per day and yields of cis9,trans11-18:2 were 8, 8, and 5 g per day, respectively, for cows fed PAM, PPM, or TMR. Responses (except for cis9,trans11-18:2 yield) were higher when PPM was fed, compared with PAM. Concentration of 18:3n−3 in milk fat from cows fed PAM or PPM increased by 60% compared with cows fed TMR only. Pasture intake also resulted in two-fold increases in concentrations of trans11,cis15-18:2, an intermediate of 18:3n−3 hydrogenation, in milk fat. Milk fat concentrations of trans11-18:1, cis9,trans11-18:2, and trans11,cis15-18:2 were positively correlated with 18:3n−3 concentration. Results suggest daily access to pasture for 8 h can substantially increase concentrations of 18:3n−3, trans11,cis15-18:2, cis9,trans11-18:2, and trans11-18:1 in milk fat of cows dependent on a TMR for the majority of their nutrients. Although desaturation of rumen-derived vaccenic acid seems to account for the majority of milk fat cis9,trans11-18:2 in grazing cows, data also showed that vaccenic acid desaturation may be independent of its concentration.