INTRODUCTION
Oxidative stress resulting from increased production of free radicals and reactive oxygen species, and/or a decrease in antioxidant defense, leads to damage of biological macromolecules and disruption of normal metabolism and physiology (Trevisan et al., 2001). When reactive forms of oxygen are produced faster than they can be safely neutralized by antioxidant mechanisms, oxidative stress results (Sies, 1991). These conditions can contribute and/or lead to the onset of health disorders in cattle (Miller et al., 1993).
The transition period is critical for the health of dairy cattle (Drackley, 1999). It has been observed that during the transition period cows can experience oxidative stress (Miller and Madsen, 1994; Formigoni et al., 1997; Ronchi et al., 2000), which may contribute to periparturient disorders (Miller et al., 1993; Brezezinska-Slebodzinska et al., 1994), and may be associated with metabolic diseases (Ronchi et al., 2000).
Studies on the effects of heat stress on oxidative status in cattle are lacking, and only preliminary reports are available. Harmon et al. (1997) reported a reduction of antioxidant activity of plasma in midlactating heat-stressed Holstein cows. Trout et al. (1998) reported no effects of heat stress on plasma concentration of vitamin E and ß-carotene or on muscle content of thiobarbituric acid reactive substances. Calamari et al. (1999) observed weak negative effects of heat stress on some plasma markers of oxidative status in midlactating cows.
To our knowledge, studies on the effects of exposure to hot environments on the oxidative status of transition dairy cows are not available. We assumed that the oxidative status of transition dairy cows might be impaired by heat stress. Oxidative stress can be monitored with several biomarkers (antioxidants and pro-oxidants) which can be assessed in plasma and/or erythrocytes (Passi et al., 2001). To verify our hypothesis, we analyzed changes in several markers of oxidative status in plasma and in the erythrocytes of transition Holstein dairy cows reared during the hot season (summer), and during a season characterized by thermoneutral conditions (spring).
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