Effects of heat stress and dietary zinc source on mammary tight junction of lactating dairy cows

Abstract Text: Dietary Zn has been shown to alter gut integrity in monogastrics under heat stress. However, the effect of Zn on mammary tight junction (MTJ) integrity in heat-stressed lactating dairy cows has not been studied. Seventy-two multiparous lactating Holstein cows (2.9±1.1 parity; 99.7±55.5 d in milk) were randomly assigned to 4 treatments with a 2×2 factorial arrangement to study the effect of environment and Zn source on performance and MTJ integrity (n=18/treatment). Treatments included two environments: cooled (CL) or not cooled (NC) and two Zn source: 75 ppm supplemental Zn as ZnCl (IOZ), or 35 ppm ZnCl + 40 ppm Zn-methionine complex (ZMC). The experiment was divided into baseline and environmental challenge phases, 84 d each. During the baseline phase, all cows were cooled (fans and misters over the freestall and feeding areas, average temperature-humidity index = 73) and fed respective dietary treatments while during the environmental challenge phase NC cows were not cooled (average temperature-humidity index = 78). Feed intake was measured daily. Milk yield was recorded at each milking (3×/d) and composition was analyzed weekly. Vaginal temperature was measured every 5 min for 4 d/wk. Milk and plasma samples were collected weekly for analyses of milk BSA and plasma lactose. Deprivation of cooling decreased DMI (P<0.01). Energy-corrected milk yield decreased (P<0.01) for NC cows relative to CL (24.5 vs. 34.1 kg/d). An interaction between environment and Zn source (P=0.04) occurred for milk fat percent as CL cows fed ZMC had lower milk fat percent relative to other groups. Relative to CL, NC cows had lower milk lactose and solids-not-fat percent (P=0.05) but higher concentration of milk urea nitrogen (P<0.01). Vaginal temperature was higher (P<0.01) in NC cows relative to CL (39.9 vs. 39.0 °C). Plasma lactose was similar between treatments at the start of the baseline phase, but increased in cows fed IOZ and was unchanged in cows fed ZMC throughout the baseline phase (Zn source × d, P=0.06). Relative to CL cows, plasma lactose tended to increase in NC cows over time (environment × d, P=0.09), indicating increased MTJ permeability, and feeding ZMC tended to decrease plasma lactose during the environmental challenge phase relative to IOZ (P=0.11).  In conclusion, removing active cooling impairs lactation performance and feeding a portion of dietary Zn as ZMC improves the integrity of MTJ as evidenced by the decreased permeability of lactose through MTJ.

Keywords: heat stress, mammary tight junction, zinc