Effects of solar radiation and feeding time on behavior, immune response and production of lactating ewes under high ambient temperature

Abstract

A 6-wk trial was performed with 40 late-lactation Comisana ewes, which were either exposed to or protected from solar radiation and fed either in the morning (EXPM, PROM) or afternoon (EXPA, PROA) during summer in a Mediterranean climate. Behavioral traits of ewes were recorded once per week from 0800 to 2000 h. Rectal temperature (RT) and respiration rate (RR) were measured twice weekly at 1430 h. The phytohemagglutinin (PHA) skin test was performed to induce nonspecific delayed-type hypersensitivity at d 10, 20, and 32 of the experiment. Jugular blood samples were taken from ewes at the beginning and at d 21 and 42 of the experiment. Ewe milk yield was recorded daily. Individual milk samples were analyzed weekly for milk composition, coagulating properties, somatic cell count (SCC) and polymorphonuclear neutrophil leukocyte counts (PMNLC) and every 2 wk for bacteriological characteristics. Solar radiation and the interaction of solar radiation x time of feeding had significant effects on rectal temperatures. EXPM ewes had higher rectal temperatures than EXPA ewes, which in turn exhibited higher RT compared with PROM and PROA ewes. EXP groups also had significantly higher respiration rates than PRO groups. Immune response was lower in EXPM ewes at d 10 and in EXPM, EXPA, and PROM animals at d 20 compared with PROA ewes. Exposure to solar radiation resulted in decreased plasma concentrations of alanine amino-transferase, alkaline phosphatase, potassium, and magnesium, as well as in increased levels of nonesterified fatty acids and aspartate amino-transferase. Milk yield and composition were not changed by exposure to solar radiation and time of feeding, but the EXPM treatment resulted in lower yields of casein and fat and reduced clot firmness compared with the three other treatments. Milk SCC was similar across treatments, but PMNLC was higher in EXPM than in PROM and PROA milk. EXPM animals also had the greatest amounts of total and fecal coliforms and of Pseudomonadaceae as well as the highest number of mastitis related pathogens in their milk. Results suggest that provision of shaded areas can play a major role in helping lactating ewes to minimize the adverse effects of high ambient temperatures on thermal balance and energy and mineral metabolism. Changing the time of feeding to late afternoon may be beneficial to exposed ewes in lowering their heat loads during the warmest hours of the day, thereby reducing the detrimental impact of thermal stress on immune function and udder health.