Effect of forage conservation method, concentrate level and propylene glycol on diet digestibility, rumen fermentation, blood metabolite concentrations and nutrient utilisation of dairy cows

The current study was conducted to establish if variations in nutrient supply due to forage conservation method could be compensated for by increases in concentrate feeding or supplements of a glucogenic substrate. Thirty two Finnish Ayrshire dairy cows were used in a cyclic change-over experiment with four 21-day experimental periods and a 4×2×2 factorial arrangement of treatments to evaluate the effects of forage conservation method, concentrate level and propylene glycol (PG), and their interactions, on digestibility, rumen fermentation, blood metabolite concentrations and nutrient utilisation. Experimental treatments consisted of four conserved forages offered ad libitum, supplemented with two levels of a cereal-based concentrate (7 or 10 kg per day) and PG (0 and 210 g per day) fed as three equal meals. Forages were prepared from primary growths of timothy and meadow fescue swards and ensiled using no additive (NA), an inoculant enzyme (IE) preparation or a formic acid (FA)-based additive or conserved as hay 1 week later. Forage conservation method had only minor effects on digestibility. Increases in concentrate feeding reduced the digestibility of neutral detergent fibre (NDF) while PG supplements tended to decrease that of nitrogen. Rumen fermentation of grass silage-based diets was characterised by higher molar proportions of butyrate and branched chain volatile fatty acids (BCVFA) and lower proportions of acetate compared with hay-based diets. Use of an ensiling additive reduced molar proportions of butyrate, while relative to FA silage, rumen fermentation of IE silage was associated with lower proportions of acetate and higher proportions of propionate and BCVFA. Increases in concentrate feeding resulted in a shift towards butyrate at the expense of acetate, while PG supplementation resulted in a rumen fermentation with higher proportions of propionate and lower proportions of acetate. Relative to silage fed animals, cows receiving hay-based diets had higher plasma concentrations of glucose and lower concentrations of insulin and beta-hydroxybutyrate (β-OHB). Increases in concentrate feeding tended to increase glucose concentrations while PG supplements elevated glucose but lowered plasma β-OHB concentrations. Differences in rumen fermentation associated with the ingestion of forages conserved using different methods were sufficient to suggest an important role on subsequent nutrient utilisation. Increases in concentrate feeding or supplements of a glucogenic precursor were unable to compensate for variations in nutrient supply due to forage conservation method.