AIMTo investigate the effects of cysteamine compound (Lactonin) on milk production of late-lactating cows during hot summer when the temperature humidity index (THI) was higher than 76 and cows were suffered from heat stress.

METHODSIn this experiment 96 black and white dairy cows, based on milk yield (M) prior to the experiment, were assigned into 4 groups (G): G1 (M < 24 kg/d), G2 (24 < M< 28 kg/d), G3 (28 < M < 32 kg/d) and G4 (M > 32 kg/d). Each group (n = 24) was further divided into subgroups of Lactonin (3000 U/d) treatment (LT, n = 49) and control (n = 47).

RESULTSIn G1 of LT, the rectal temperature decreased (P < 0.05), milk yield, fat-corrected milk, milk fat and feed conversion rate (FCR) increased (P < 0.05). These were companied with trendy of higher milk protein and lower somatic cell count. With whole LT cows (n = 49), the mean milk fat (%) increased (P < 0.05), mean milk protein tended to increase, and the mean milk yield and FCM tended to be enhanced. Plasma T3, T4 tended to decline whereas insulin enhanced (P < 0.01) significantly in LT herd (n = 49).

CONCLUSIONLactonin helps heat-stressed cow to maintain a more normal metabolism in hot summer. This positive effect of Lactonin on cow performance is associated with Lactonin-dependent alteration of plasma insulin, T3 and T4.