Ruminal and blood responses to propylene glycol during frequent feeding

The objective of the current experiment was to study the responses of ruminal and blood metabolites of Holstein dairy cows to propylene glycol (PG) under different methods of delivery during frequent feeding. By providing the same amount (200 mL or 200 g) of PG, delivery methods for PG were assessed: 1) control treatment: no PG; 2) dietary treatment: 200 g of PG as a dry product (65% purity; corresponded to 308 g of the dry product) mixed into the TMR; 3) oral-drench treatment: 200 mL of liquid PG (100% purity) orally drenched; and 4) rumen-drench treatment: 200 g of PG as a dry product drenched via the rumen cannula to mimic top dressing. Eight multiparous (lactation = 3 +/- 1.1 SD) ruminally cannulated Holstein dairy cows (DIM = 204 +/- 104.5 SD) were fed PG for 4 d (d 11 to 14) in a replicated 4 x 4 Latin square design with an experimental length of 14 d for each period. On the last day of each period, serial blood samples were removed from an indwelling catheter placed in the right jugular vein immediately before and for 4 h after PG administration. Cows were fed at 12x feeding/d for 2 d before entering the serial sampling period to minimize postprandial influences on blood metabolites. Ruminal content was also sampled hourly for 4 h on d 14. Milk was sampled from 2 consecutive milkings on d 13 during each period. Dry matter intake and milk yield were not affected by PG. Percentages of milk lactose were increased by PG delivered by all methods tested in the current experiment. Ruminal concentrations (as percentages of total volatile fatty acids) of acetate were decreased and concentrations of propionate and isovalerate were increased by PG, regardless of the delivery method; however, total volatile fatty acid concentration was not affected by PG. Ruminal concentrations of butyrate were decreased and concentrations of valerate were increased by PG drench, via either an oral or ruminal drench. The degree of reduction in butyrate concentration or increase in valerate concentration was affected by PG dose. Serum insulin peaked more rapidly and at a greater concentration for cows receiving PG via drenching, but not when PG was provided as a part of the TMR. Plasma glucose, however, tended to peak more rapidly at a greater concentration for cows receiving PG, regardless of the delivery method. Propylene glycol for the amount drenched (orally or ruminally) or fed (incorporated into the ration) shifted ruminal fermentation toward a more glucogenic environment. Drenching demonstrated a better efficacy than feeding PG because of the amount of PG that was available to the animal at the time of sampling. Effects of drenching dry PG into the rumen were comparable with orally drenching liquid PG.