Effects of heat stress on energetic metabolism in lactating Holstein cows

Abstract

Heat stress has an enormous economic impact on the global dairy industry, but the mechanisms by which hyperthermia negatively affect systemic physiology and milk synthesis are not clear. Study objectives were to evaluate production parameters and metabolic variables in lactating dairy cows during short-term heat stress or pair-fed conditions coupled with bST administration. Twenty-two multiparous Holstein cows were subjected to 3 experimental periods: 1) thermoneutral conditions with ad libitum intake for 7 d (P1); 2) heat stress (HS) with ad libitum intake (n=10) or pair-fed (PF) in thermoneutral conditions (n=12) for 7 d (P2), and 3) 7 d of HS or PF in conditions as described in P2 with recombinant bovine somatotropin administered on d 1 (P3). All cows received an intravenous glucose tolerance test (GTT) on d 5 of each period. Heat stress conditions were cyclical and temperatures ranged from 29.4 to 38.9 degrees C. Rectal temperatures and respiration rates increased during heat stress (38.6-40.4 degrees C and 44-89 breaths/min, respectively). Heat stress reduced dry matter intake by 30% and by design PF cows had similar intake reductions (28%). During heat stress and pair-feeding, milk yield decreased by 27.6% (9.6kg) and 13.9% (4.8kg), respectively, indicating that reduced feed intake accounted for only 50% of the decreased milk production. Milk yield increased with recombinant bovine somatotropin in both HS (9.7%) and PF (16.1%) cows. Cows in both groups were in positive energy balance (3.95 Mcal/d) during P1 but entered negative energy balance during P2 and P3 (-5.65 Mcal/d). Heat stress and pair-feeding treatments decreased (9.3%) basal glucose concentrations. Heat stress conditions had no effect on basal NEFA levels during P2; however, PF cows (despite a similar calculated energy balance) had a 2-fold increase in basal NEFA concentrations. Both groups had increased plasma urea nitrogen levels during P2 and P3 compared with P1. Basal insulin levels increased (37%) during P2 and P3 in HS cows but did not differ between periods in PF cows. During P2 and compared with P1, PF cows had a decreased rate of glucose disposal, whereas HS cows had a similar disposal rate following the GTT. During P2 and compared with P1, PF cows had a reduced insulin response whereas HS cows had a similar insulin response to the GTT. In summary, reduced nutrient intake accounted for only 50% of heat stress-induced decreases in milk yield, and feed intake-independent shifts in postabsorptive glucose and lipid homeostasis may contribute to the additional reduction in milk yield.

Metabolic and hormonal acclimation to heat stress in domesticated ruminants

“…Despite marked reductions in nutrient intake, heat-stressed cattle exhibit increased basal insulin levels and stimulated insulin response (Itoh et al, 1998; Bernabucci, Lacetera, Baumgard, Rhoads, Ronchi and Nardone 1172 Baumgard and Rhoads, 2007;Wheelock et al, 2010), and this agrees with heat-stressed rodent experiments (Torlinska et al, 1987). The increased basal and stimulated insulin levels may explain the lack of an increase in basal NEFA levels in heatstressed cows as insulin is a potent antilipolytic hormone (Vernon, 1992).…”

Heat stress and ACTH administration on cortisol and insulin-like growth factor I (IGF-I) levels in lactating Holstein cows

“…Some authors demonstrated that IGF-I had a slight decrease during heat stress (Sarko et al 1994;Rhoads et al 2009;Wheelock et al 2010). In the present study, however, IGF-I concentrations were not influenced by ACTH administration or increase in plasma CORT concentration.…”

“…In the same way, while some authors have reported that growth hormone (GH) and insulin-like growth factor I (IGF-I) are negatively influenced by heat stress (Sarko et al 1994;Rhoads et al 2010;Wheelock et al 2010), others mentioned that IGF-I levels during heat stress are similar to those obtained within thermal comfort zone (McGuire et al 1991;Hirayama et al 2004;Chaiyabutr et al 2008;Collier et al 2008). For these reasons, the interactions between heat stress and CORT and between IGF-I and environmental heat remain controversial.…”

Dairy cattle in a temperate climate: the effects of weather on milk yield and composition depend on management

“…Similar to milk yield, fat content was highest at moderate WSs. Most previous studies also report a decrease in the proportion of fat in milk (Bouraoui et al, 2002;Hammami et al, 2013;Smith et al, 2013) or total milk fat (Lambertz et al, 2014) under conditions of heat stress or increasing temperature, although others found no effect (Knapp and Grummer, 1991;Wheelock et al, 2010). While an increase in the number of sunshine hours was associated with an increase in milk yield in cows outdoors and a decrease in milk yield in cows indoors, the inverse was true for fat content.…”