Abstract
High ambient temperatures have severe adverse effects on biological functions of high-yielding dairy cows. The metabolic adaption to heat stress was examined in 14 German Holsteins transition cows assigned to two groups, one heat-stressed (HS) and one pair-fed (PF) at the level of HS. After 6 days of thermoneutrality and ad libitum feeding (P1), cows were challenged for 6 days (P2) by heat stress (temperature humidity index (THI) = 76) or thermoneutral pair-feeding in climatic chambers 3 weeks ante partum and again 3 weeks post-partum. On the sixth day of each period P1 or P2, oxidative metabolism was analyzed for 24 hours in open circuit respiration chambers. Water and feed intake, vital parameters and milk yield were recorded. Daily blood samples were analyzed for glucose, β-hydroxybutyric acid, non-esterified fatty acids, urea, creatinine, methyl histidine, adrenaline and noradrenaline. In general, heat stress caused marked effects on water homeorhesis with impairments of renal function and a strong adrenergic response accompanied with a prevalence of carbohydrate oxidation over fat catabolism. Heat-stressed cows extensively degraded tissue protein as reflected by the increase of plasma urea, creatinine and methyl histidine concentrations. However, the acute metabolic heat stress response in dry cows differed from early-lactating cows as the prepartal adipose tissue was not refractory to lipolytic, adrenergic stimuli, and the rate of amino acid oxidation was lower than in the postpartal stage. Together with the lower endogenous metabolic heat load, metabolic adaption in dry cows is indicative for a higher heat tolerance and the prioritization of the nutritional requirements of the fast-growing near-term fetus. These findings indicate that the development of future nutritional strategies for attenuating impairments of health and performance due to ambient heat requires the consideration of the physiological stage of dairy cows.
Highlights
Ambient temperatures above 21°C cause serious heat stress in high-yielding dairy cows [1]
By design, feed offered to PF animals during period 2 (P2) was restricted by the extent the HS cows reduced their voluntary dry matter intake (DMI) in P2, resulting in a mean DMI of 62.3% relative to period 1 (P1) for PF cows in the ap and pp stage (Fig 1A, Table 3)
As compared to P1, HS cows responded to environmental heat by an increased rectal temperature and respiratory rate, but significant alterations of heart rate were only apparent in the pp stage
Summary
Ambient temperatures above 21°C cause serious heat stress in high-yielding dairy cows [1]. This leads to impairments of animal health, milk yield and reproduction [1,2], resulting in estimated economic losses of 897 million dollars in the USA, despite intensive cooling, every year [3]. In view of the higher frequency of extreme environmental heat events and increasing average temperatures predicted for the decades [4], managing hot ambient temperatures will become an issue even for dairy farmers in areas of moderate climates [2,5]. Serious depressions of feed intake and milk yield will occur [7]
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