Effect of heat stress during the dry period on mammary gland development

Discipline: stress; Key words: heat stress, dry period, mammary gland, epithelial cell

With a warmer future climate, heat stress is expected to occur much more frequently than today. Heat stress put pressure on various metabolic and physiological functions of the cow, resulting in lower milk yields, impaired reproduction and various health-related problems because of a compromised immune system. The lower milk production does not only result because of lower feed intake as heat stress during the dry period also affects milk yield in the subsequent lactation. This was the topic investigated by Dr S Tao and co-workers. The title of the paper is:Effect of heat stress during the dry period on mammary gland development,and it was published in the Journal of Dairy Science, Volume 94 of 2011, pages 5976 to 5986.

Heat stress during the dry period negatively affects liver metabolism and cellular immune function during the transition period, and as mentioned, milk production in the subsequent lactation. However, the cellular mechanisms involved in the depressed mammary gland function remain unknown. Therefore, the objective of the study by Dr Tao and colleagues was to determine the effect of heat stress during the dry period on various indices of mammary gland development in cows having had more than one lactation.

Cows for the experiment were dried off approximately 46 days before expected calving and randomly assigned to two treatments: heat stress (HT) or cooling (CL), based on mature equivalent milk production. Cows in the CL treatment were provided with sprinklers and fans that came on when ambient temperatures reached 21.1°C, whereas HT cows were housed in the same barn without fans and sprinklers. After calving, all cows were housed in a free-stall barn with cooling. Rectal temperatures were measured at 07:30 and 14:30 and respiration rates recorded at 15:00 on a Monday-Wednesday-Friday schedule from dry off to calving. Milk yield and composition were recorded daily up to 280 days in milk. Daily dry matter (feed) intake was measured from dry off to 42 days relative to calving. Mammary gland biopsies were collected at dry off, 20 days before calving, 2 days after calving and 20 days after calving. Specific antigens and enzymes were used to evaluate mammary cell development, proliferation and cell death.

Heat-stressed (HT) cows had higher rectal temperatures in the morning (38.8 vs. 38.6°C) and afternoon (39.4 vs. 39.0°C), greater respiration rates (78.4 vs. 45.6 breath per minute), and decreased dry matter intake (8.9 vs. 10.6 kg per day) when dry compared with CL cows. Relative to HT cows, CL cows had greater milk production (28.9 vs. 33.9 kg per day), lower milk protein concentration (3.01 vs. 2.87%), and tended to have lower somatic cell score (3.35 vs. 2.94) through 280 days in milk. Heat stress during the dry period decreased mammary cell proliferation rate (1.0 vs. 3.3%) at 20 days before calving in HT compared with CL cows. Mammary cell death was not affected by dry period heat stress.                    

The authors concluded that heat stress during the dry period compromises mammary gland development before calving, which decreases milk yield in the next lactation.