Abstract
The aim of this study is to determine correlation between change in milk production and metabolic adaptation of cows during heat-stress. Experiment included 30 Holstein-Frisian cows. Cows exposed to heat-stress, which is expressed in high THI index, have lower milk production, higher concentration of insulin and lover concentrations of glucose and NEFA. Indexes of insulin resistance-RQUICKI, insulin: glucose and insulin: NEFA relations are higher in cows under heat stress. Grouping cows in accordance to reduced milk yield have showed that trend of reduction of glucose and NEFA levels, followed by increased insulin level, higher RQUICKI, greater insulin: glucose and insulin: NEFA ratios, started and increased during the reduction in milk production. Among this, significant correlation was founded between these parameters and milk yield. Significant correlation was also noted between those parameters. Change in metabolic value is of great importance for prediction of cows that would have decreased milk yield (above 18%). With almost 90% certainty cows with great reduction in milk yield can be detected. These cows have had following relative changes in values of metabolic parameters during heat-stress compared to thermo-neutral period: insulin increased for ≥12.5%, reduction in NEFA values for ≤14.1%, reduction in glucose for 21.5%, RQUICKI index increased for ≥9.6%, insulin: NEFA ratio increased for ≥20.1% and insulin: glucose for ≥20.3%. Dynamical changes in metabolites and insulin resistance values have great influence on milk yield in cows under heat stress. Decreasing in glucoses followed by increased insulin level and increased insulin sensitivity indicate that glucose is transferred from udder to other tissues which can cause decreased milk production.
Highlights
Heat stress represents state in which animal is exposed to high ambient temperatures
Because of that heat stress of cows is characterized with reduced milk production and changed composition of milk (Cincović and Belić, 2009)
In high selected dairy cows lipolysis allows greater usage of fats in regular energetic and metabolic processes and glucose is being saved for milk production
Summary
Heat stress represents state in which animal is exposed to high ambient temperatures These temperatures are out of biological optimum. Because of those conditions, heat production by animal is greater than heat release. Reduced feed intake causes decrease in glucose concentration in peripartal and lactating period. Cows during heat stress have reduced feed intake but lipolysis and ketogenesis are reduced. Ventilation of animals in heat stress results in stabilization of heat stress caused metabolic effects, concentrations of glucose and NEFA are increased and values of hormones and insulin sensitivity indexes are changed (Cincović et al, 2013). In high selected dairy cows lipolysis allows greater usage of fats in regular energetic and metabolic processes and glucose is being saved for milk production. In order to that reduced insulin production and insulin resistance occur
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