In two consecutive summers, 21 and 18 cows, respectively, were monitored for blood parameters and milk yield, to assess their variation according to the level of heat stress at different stages of lactation. During both years, the cows were monitored according to their lactation phase (early, mid-, and late) at the beginning of the summer. Climatic conditions were described through the temperature humidity index. Cows were monitored for breathing rate, rectal temperature, blood metabolites and enzymes, and milk yield. In the first year, two hotter periods were identified, with more severe conditions in the second one, when cows had rectal temperatures higher than 40 degrees C. In the second year, only one hotter period was identified, with a heat stress comparable to that of the first period of the first year. Milk yield declined during the hotter period; in both years, a higher reduction in milk yield was recorded for cows in the mid-stage of lactation. The decrease in plasma glucose during the hotter period was evident in both years. Plasma cholesterol also decreased in those periods. The reduction in plasma ALP activity in the hotter period of both years confirmed the role of this enzyme as a quick and reliable heat-stress blood marker.

Two trials were conducted to evaluate the effect of moderate (0.7 kg) and accelerated (0.9 kg) average daily gain before (trial 1) and after (trial 2) puberty on body condition, metabolic profile, and first lactation milk production of Italian Holstein-Friesian heifers. There were 20 heifers in trial 1 and 22 in trial 2. Trials started when heifers averaged 150 and 300 kg of body weight in trial 1 and 2, respectively, and lasted 7 mo (experimental period). Across diet groups, half of the heifers were mated at first estrus after 370 kg and the other half after 420 kg of body weight gain. Actual average daily gains were 0.667 and 0.775 kg in trial 1 and 0.748 and 0.824 kg in trial 2 for moderate and accelerated experimental groups, respectively. Diets for high average daily gain did not affect body condition during growing phase in trial 1, whereas it did in trial 2. High average daily gain increased plasma glucose in trial 1 and plasma urea concentration in trial 2. Rearing diet did not affect milk production and milk protein percent; age in both trials. High average daily gain decreased milk fat percentage in trial 2. Early calving negatively influenced milk production in both trials and milk fat percentage in trial 1. Early calving heifers showed higher protein percentage than those with late calving only in trial 1.

Forty-two dairy cows were subdivided in three groups of 14 cows each. Each pen of cows was provided with a feeding area, a freestall area, and an external paddock. The study was conducted in an experimental barn located in the Po Valley of Italy. The cows were observed during summer 1995 for milk yield and quality and for their behavior (movements and residence time in the different areas of each pen) under different microclimatic conditions (temperature, relative humidity, light exposure, air speed). The feeding and freestall areas were arranged with: (1) fans in the first pen; (2) fans and misting in the second pen; and 3) the third pen was used as control. During the summer months (from June to September) microclimatic conditions, milk yield, and cow position (monitored with video cameras and automatic still cameras placed in different areas of the barn) were recorded. During the hottest period, lower milk yield reductions were recorded for the cows housed in pens with fans or with fans and misting. This result was consistent with the behavior of the cows inside experimental pens that spent longer periods of resting time in the feeding and freestall areas, and showed overall behavior similar to that observed during a cooler period. Some cows also lay in the wet dirty areas of the pens or crowded in a well-ventilated place. The results of this study confirm the usefulness of cooling systems for cows in a warm climate.

In two consecutive summers, 21 and 18 cows respectively were monitored for acid-base chemistry and some blood minerals, to assess their variation according to the level of heat stress at different stages of lactation. During both years, the cows were monitored according to their lactation phase (early, mid-, and late) at the beginning of the summer. Climatic conditions were described through the temperature humidity index. Cows were monitored weekly for: breathing rate, rectal temperature, hemogas parameters and blood minerals (morning and afternoon collection). In the first year, two hotter periods were identified, with more severe conditions in the second one, when cows had rectal temperatures higher than 40 degrees C. In the second year, only one hotter period was identified, with a heat stress comparable to that of the first period of the first year. The behaviour of rectal temperature, breathing rate and the parameters of the acid-base status indicated that the suffering of the cows was on the borderline between mild and high heat stress during the hotter periods only, according to the climatic conditions in the two years. During the hotter periods, the acid-base chemistry differed significantly with a reduction of HCO3- and an increase of Cl during the hotter hours of the day. The compensation mechanism for mild alkalosis during hotter hours maintained blood pH and the HCO3- returned to normal values during the night. Significant reductions were observed for Mg and Zn during the hotter periods. The cows in late lactation appeared to be less stressed by the hot climate.

No abstract