Effect of zinc sources on milk yield, milk composition and plasma concentration of metabolites in dairy cows

Discipline: minerals; Keywords: milk composition, milk production, metabolism, somatic cell count.

Zinc (Zn) is an essential trace mineral and plays an important role in DNA and RNA synthesis and replication and in cell proliferation. Zinc supplementation has been shown to increase milk yield, live weight and growth rate, and enhanced disease resistance. With reference to the latter, it has been suggested that Zn deficiency could result in increased somatic cell count (SCC) and ultimately increase mastitis in dairy cows. 

Supplemental Zn in rations for dairy cows is usually in the form of inorganic Zn [zinc oxide (ZnO) or zinc sulfate (ZnSO4)]. Some studies, however, have reported that organic Zn is more readily absorbed by ruminants than inorganic Zn. Thus, use of trace elements from organic sources (that is, complexed chelated amino acids, proteinates), which are more bioavailable compared with those from inorganic sources, can be nutritionally important for maximizing milk production and maintaining health. Recently, a novel source of elemental Zn has become available for animal diets. The development of nanotechnology has led to the creation of Zn nano particles (ZnN) which have unique properties including great specific surface area, high surface activity, high catalytic efficiency and strong absorbing ability. The feeding of ZnN has been shown to have greater efficacy and reduced toxicity compared to conventional Zn sources. In addition, it appears to have superior antibacterial properties when compared to Zn from conventional sources. Thus, the objective of the study by Dr S. Bakhshizadeh and colleagues was to evaluate the effects of three sources of Zn on feed intake, milk yield, milk composition, plasma concentration of metabolites and changes in body condition score (BCS) in lactating Holstein dairy cows. They have published their results in the South African Journal of Dairy Science, Volume 49 of 2019, page 882 to 889. The title of the paper is: Effect of zinc sources on milk yield, milk composition and plasma concentration of metabolites in dairy cows         

Twenty-four dairy cows were randomly allocated to one of four treatments in a randomized complete block design. The treatments consisted of i) control diet (no zinc supplement), ii) zinc oxide (ZnO), iii) zinc glycine (ZnGly), and iv) zinc nano (ZnN). The Zn sources were added to provide 60 mg of supplemental Zn per kg diet. Milk yield was recorded automatically at each milking, and the composition of the milk from one 24-hour period was analysed weekly with additional samples being taken fortnightly for SCC. Samples for SCC determination were analysed within 24 hours. Body condition score was scored each week based on a 5-point scale with 0.25 intervals, where 1 = thin and 5 = fat. During weeks 0, 4, 8, and 12 of the study, blood samples were collected two hours after the morning feeding for determination of enzyme activity, Zn concentration and biochemical parameters.

There were no differences between treatments Control, ZnO, ZnGly and ZnN respectively in dry matter intake (22.8, 22.8, 23.2 and 23.0 kg/day; p=0.39), milk yield (37.9, 37.3, 38.7 and 38.3 kg/day; p=0.07), bodyweight (632, 642, 641 and 638 kg; p=0.67) and body condition score (3.31, 3.32, 3.24 and 3.26; p=0.73) of the cows between treatments, although milk yield tended (p=0.07) to be higher in the ZnGly and ZnN treatments. Zinc supplementation in the form of ZnN and ZnGly decreased SCC compared with the other treatments (153.7, 150.0, 142.5 and 140.2 103/ml; p=0.01). The superoxide dismutase and plasma Zn concentrations in the cows provided by ZnGly (2170 iu/ml) and ZnN (2187 iu/ml) were greater (p=0.02) than those in the ZnO (2149 iu/ml) and control groups (2146 iu/ml). No difference was detected between groups in biochemical and haematological parameters. However, blood urea nitrogen concentrations of cows supplemented with ZnGly (15.9mg/dl) and ZnN (15.5mg/dl) were less (p=0.006) than for the ZnO (16.5mg/dl) supplemented and control cows (17.0mg/dl). 

The results suggested that nano and organic Zn sources in the diet of dairy cows should be more suitable than inorganic Zn as supplements for dairy cows.