Abstract: 【Objective】 The aim of this study was to explore the effects of supplemental light on blood endocrine hormone concentration and circadian rhythm change in late-gestation Holstein breeding dairy cows,so as to provide a theoretical basis for large-scale and standardized light management of late-gestation Holstein breeding dairy cows under supplemental light. 【Method】 Twenty-four Holstein breeding dairy cows with similar body weights (600 kg±5 kg) in one months before parturition were selected and randomly divided into a natural light group and a supplemental light group using a randomized experimental design.Dairy cows in natural light group received natural light for 10-11 h.Based on natural light,Dairy cows in supplemental light group were supplemented with fluorescent light at 04:00-08:00 and 16:00-20:00 every day,so that the light received 16 h.During the experiment,cows had free access to food and water,and the experimental period was 63 d.Five cows in each group were selected for caudal root blood sampling at six time points (03:00,07:00,11:00,15:00,19:00,and 23:00) on the 63rd day of the experiment.Indicators such as melatonin (MT),growth hormone (GH),insulin-like growth factor-Ⅰ(IGF-Ⅰ),leptin (LEP),prolactin (PRL),and cortisol (COR) in serum were measured.At the same time,a cosine mathematical model was used to fit the changes in hormone concentrations during the day and night to observe the circadian rhythm changes of each hormone. 【Result】 ①At each sampling time point,compared with natural light group,there were no significant differences of the concentrations in various serum hormones of dairy cows in supplemental light group (P＞0.05),but the mean concentrations of MT,IGF-Ⅰ,PRL,and COR in serum of dairy cows in the supplemental light group were higher than that in natural light group,whereas the mean concentrations of GH and LEP were lower than that in natural light group.②The circadian changes of MT,GH,LEP,PRL,and COR concentration in serum of dairy cows in both groups did not show a cosine oscillation rhythm curve change (P＞0.05).IGF-Ⅰ concentration in serum of dairy cows in natural light group had circadian rhythmic expression (R²＞0.5 and P＜0.05).The concentrations of MT，GH,IGF-Ⅰ,LEP,PRL,and COR in serum of dairy cows in natural light group exhibited superior goodness-of-fit in circadian rhythm patterns compared with supplemental light group. 【Conclusion】 Supplemental light exerted effects on the rhythmicity of MT,PRL,and COR concentrations in serum of late-gestation Holstein breeding dairy cows,and altered the circadian rhythmic expression of IGF-Ⅰsecretion.Therefore,it was recommended to appropriately increase the light to improve the circadian rhythm of late-gestation Holstein breeding dairy cows,the physiological state and production performance of dairy cows could be improved by regulating the biological clock and hormone secretion.

Key words: late-gestation; supplemental light; Holstein breeding dairy cows; blood hormones; circadian rhythms