DHI (Dairy Herd Improvement) is a standard measurement used to evaluate the milk productive performance of the dairy cows [1]. DHI measurement includes the parameters of milk somatic cell number (SCC), milk yield, protein, fat, lactose, dry matter, urea nitrogen of the milk [2]. DHI establishment is an important approach to improve dairy herd performance and to breed high-yielding cow herds [3]. Among the parameters measured in the DHI, the SCC is a most important indicator to reflect the milk quality and the status of breast health of the cows [4]. Many factors influence the milk SCC. These include environmental stresses, number of fetuses, air humidity, feeding management, nutrition level, ect. [5, 6]. Among the environmental stresses, the seasonal alterations have a significant impact on the SCC [7]. For example, high temperature (hot stress) in summer often causes reduced milk production and elevated SCC [8]. Cows in different ages exhibit variable DHI indexes[9]. This partially relates to the fact that cows are sexually matured at the first mating time, but they still do not reach fully physical maturity, thus, with the continuous body development, the somatic cell score exhibits regular changes with the gestational times[10]. In addition, cows in the different lactation periods including early, peak, middle and late lactation periods have different milk production and physiological characteristics which also result in variable DHI values. For example, the SCC in the early stage of lactation is higher than that in the peak and middle stages of lactation[11].
SCC, lactose, fat, protein and urea nitrogen indexed in the DHI determine milk quality[12]. The higher the SCC, the lower the milk quality is[13]. When the milk SCC exceeds 200,000 /mL, the cow will be diagnosed as the mastitis. The extremely high SCC always indicates the serious breast infection of the cows[14]. Lactose is a unique ingredient in milk and is present in many infant formula and dairy products[15]. Milk fat is a complex mixture of different types of fat which is the main source of energy of the milk[16]. The milk protein is mainly casein, with small amounts of albumin and globulin[17] which is an important protein source of human diet. Urea nitrogen reflects the efficiency of nitrogen and protein metabolism in rumen. The urea nitrogen content provides accurate information as to how much milk should be consumed based on the daily protein diet requirement[18]. Changes in any of these factors mentioned above will affect the quality of raw milk and the quality of its processed products[19]. The milk with balanced composition ratio of these factors is considered to have suitable nutritional value [20].
To produce high quality milk with low SCC is a priority agenda for dairy famers as well as researchers. In the previous study, our group have found that melatonin application reduces milk SCC[21] and this study is the continuation of the previous work to investigation whether melatonin administration can also increase the milk nutritional value. This aspect has not been reported till now.
Melatonin (MT) is originally classified as an endocrine hormone mainly produced by the pineal gland in vertebrates. Currently, it is found that many cells, organs and tissues have the capacity to synthesize melatonin. These include gastrointestinal tract[22], retina[23] bone marrow [24], skin[25], oocytes[26], etc. The main sites of melatonin synthesis are the mitochondria[27] and thus, actually all cells appear to have the ability to synthesis melatonin. Melatonin production exhibits an apparent circadian rhythm with the peak at dark and baseline level during the day[28]. Therefore, melatonin has been used to mimic the photoperiod signal to improve milk yield[29]. Few studies have reported the effects of melatonin supplement on milk quality in cows evaluated by DHI which is considered as the best index to reflect the milk quality in the field. In the current study, thus, we have adopted DHI to systemically evaluate melatonin’s effects on the milk quality. The results are reported following.