Milk Production and Energetic Metabolism of Heat-Stressed Dairy Goats Supplemented with Propylene Glycol

SIMPLE SUMMARY: Heat stress (HS) causes significant milk production losses and negatively impacts animal health. Previous research indicated that heat-stressed goats eat less and degrade their skeletal muscles to cover shortages in nutrient requirements. Propylene glycol (PG) is a glucogenic precursor and may be a remedy for energy shortage in HS situations. In the current study, dairy goats were exposed to thermoneutral (TN; 15 to 20 °C) or HS (12 h/d at 37 °C and 12 h/d at 30 °C) conditions. In each ambient temperature, goats were fed a control diet (CO) or the same diet supplemented with PG. Goats supplemented with PG gained more body weight, and experienced greater blood glucose and insulin levels compared to CO goats. However, supplementation with PG depressed feed intake and milk fat content. In conclusion, feeding propylene glycol was useful to reduce body weight losses typically observed under HS conditions, but did not improve milk yield or milk composition. ABSTRACT: Heat-stressed dairy animals increase their reliance on glucose. This elevated glucose demand is partially met by increasing the conversion of glucogenic amino acids (AA) in the liver. Propylene glycol (PG) is a glucogenic precursor and was not tested in dairy goats under thermoneutral (TN) and heat stress (HS) conditions simultaneously. We hypothesize that if HS-goats are fed with PG, they would get more glucose and consequently spare more glucogenic AA for milk protein synthesis rather than gluconeogenesis. Eight multiparous dairy goats (40.8 ± 1.1 kg body weight; 84 ± 1 days in milk) were used in a replicated 4 × 4 Latin square design of 4 periods; 21 d each (14 d adaptation, 5 d for measurements, and 2 d of transition). Goats were allocated to one of 4 treatments in a 2 × 2 factorial arrangement. Factors were control (CO) without PG or 5% of PG, and thermoneutral (TN; 15 to 20 °C) or heat stress (HS; 12 h/d at 37 °C and 12 h/d at 30 °C) conditions. Feed intake, rectal temperature, respiratory rate, milk yield, milk composition, and blood metabolites were measured. Compared to TN, HS goats had lower (p < 0.01) feed intake (–34%), fat-corrected milk (–15%), and milk fat (–15%). Heat-stressed goats also tended (p < 0.10) to produce milk with lower protein (–11%) and lactose (–4%) contents. Propylene glycol increased blood glucose (+7%; p < 0.05), blood insulin (+37%; p < 0.10), and body weight gain (+68%; p < 0.05), but decreased feed intake (–9%; p < 0.10) and milk fat content (–23%; p < 0.01). Furthermore, blood non-esterified fatty acids (–49%) and β-hydroxybutyrate (–32%) decreased (p < 0.05) by PG. In conclusion, supplementation of heat-stressed dairy goats with propylene glycol caused milk fat depression syndrome, but reduced body weight loss that is typically observed under HS conditions. Supplementation with lower doses of PG would avoid the reduced feed intake and milk fat depression, but this should be tested.