Propylene glycol for dairy cows

Abstract

Abstract The objective of this review was to describe how propylene glycol (PG) is metabolised in the rumen and liver and how it affects metabolites, hormones, liver composition, feed intake and milk production, in order to evaluate whether PG is likely to prevent excessive fat mobilisation and imbalances in carbohydrate and fat metabolism and thereby reduce the risk of ketosis. PG decreases the molar ratio of acetate to propionate in rumen volatile fatty acids (VFA) because part of PG is metabolised to propionate in the rumen. The remaining PG is absorbed directly from the rumen without alteration and enters gluconeogenesis via pyruvate. Oral administration of PG increases insulin by 200–400% within 30 min after drenching, indicating that PG is absorbed rather quickly. Allocation of PG also increases plasma glucose, although the response is limited, probably because of the large increase in insulin. PG decreases plasma concentrations of non-esterified fatty acids (NEFA) and beta-hydroxybutyrate (BHB), especially in early lactating cows with relatively high levels of NEFA. PG also reduces the triacylglycerol (TG) content of the liver and the concentrations of ketone bodies in milk and hence, has anti-ketogenic properties. Factors, such as metabolic status of the animals, time of blood sampling in relation to time of feeding PG, supplementation method and dose of PG influence the magnitude of the response in blood parameters. Generally, PG has no statistically significant effect on milk production, but for cows in early lactation PG tends to increase milk yield and reduce milk fat percentage, while milk protein percentage is unchanged. Thus, PG has no effect on energy corrected milk yield (ECM). In general, PG does not affect feed intake. However, due to its low palatability it may reduce feed intake if not mixed thoroughly with other feed components or drenched. PG may improve reproduction efficiency but more studies are needed to confirm this. PG has some side effects including ataxia, salivation, hyperventilation, and depression. However, only some cows show these signs and great variation in susceptibility is likely. The reducing effect of PG on NEFA, hepatic TG concentrations, and milk fat content indicate that PG increases energy balance of cows in early lactation. Together with the anti-ketogenic properties of PG, this suggests that PG may reduce the risk of subclinical and clinical ketosis.