Abstract
The aim of this study was to determine the effect of propylene glycol on milk yield, milk composition, parameters of subclinical ketosis and reproductive performance of high producing dairy cows. Twenty Holstein cows in the close up period were distributed into two equal groups. The control group received the basal diet. The other group was supplemented with propylene glycol (PG) (300 g per cow per day). The dietary treatment lasted from 3 weeks prepartum to 8 weeks postpartum. Body condition scores (BCS) were monitored using 1 to 5 scale at the start, parturition, 4 weeks and 8 weeks postpartum. All the cows were kept in tie-stall barns. Milk yield was daily recorded for each cow. Milk samples were collected weekly and analyzed for composition and somatic cell count (SCC). Blood samples were collected from all cows on day 10 prepartum to estimate non- esterified free fatty acids (NEFA). Further blood samples were collected on day 30 postpartum to determine serum NEFA, beta hydroxyl butyric acid (BHBA), urea, triglycerides and liver enzymes. Also, on day 30 postpartum, milk samples were collected to measure BHBA. Conception rate, days open and service pre conception were recorded for all cows. Cows supplemented with PG significantly had better BCS (3.2±0.06) than the control cows (2.8±0.04) at 8 weeks post-partum. PG significantly increased milk yield by 32.26% and milk protein by 32.9% than the control, while the other milk constituents and SCC were nearly similar in the two groups. Also, PG improved the energy status of the cows through increasing blood glucose level (57.8 ±1.1 versus 30±3.1) mg/dl and decreasing prepartum NEFA (0.3±0.02 versus 0.42±0.04) mmol/l. Also, lowering post-partum BHBA (0.73±0.12 versus 1.16±0.23) mmol/l. Moreover, PG significantly improved the reproductive performance of the lactating cows through shortening of days open by 31.0%, and decreasing service per conception by 23.08%. Finally, practical dietary supplementation of PG could be used to increase milk yield, improve reproductive performance and prevent subclinical ketosis of Holstein cows.

Key words: Propylene glycol, Holstein cows, Milk yield, Reproductive performance, Ketosis.