Eitam, H., Institute of Animal Science, Department of Ruminant Science and Genetics, Newe Ya'ar Research Center, P.O. Box 1021, Ramat Yishay 30095, Israel, Department of Evolutionary and Environmental Biology, Faculty of Science and Science Education, University of Haifa, Haifa 31905, Israel
Brosh, A., Institute of Animal Science, Department of Ruminant Science and Genetics, Newe Ya'ar Research Center, P.O. Box 1021, Ramat Yishay 30095, Israel
Orlov, A., Institute of Animal Science, Department of Ruminant Science and Genetics, Newe Ya'ar Research Center, P.O. Box 1021, Ramat Yishay 30095, Israel
Izhaki, I., Department of Evolutionary and Environmental Biology, Faculty of Science and Science Education, University of Haifa, Haifa 31905, Israel
Shabtay, A., Institute of Animal Science, Department of Ruminant Science and Genetics, Newe Ya'ar Research Center, P.O. Box 1021, Ramat Yishay 30095, Israel

Selection for higher production rate in cattle inhabiting challenging habitats may be considered disadvantageous because of possible deleterious effects on immunity and reproduction and, consequently, on calf crop percentage. In Israel, free-grazing high productive beef cows experience reduction in nutritional quality of forage during up to 8 months of the year. As milk production by dams dictates calf performance, dam's nutritional needs and rebreeding rates, the aim of the present study was to test how lactating beef cows deal with combined caloric and protein stress both at the productive and self protective levels. For this purpose, we studied the effect of long-term caloric stress on milk characteristics and gene expression of stress and milk components producing proteins. Lactating dams responded to caloric stress by decreased body weight, milk, and milk protein production. To compensate for total energy loses in milk, they produced milk of higher fat concentration and shifted the proportions of its fatty acids towards long and unsaturated ones. This was reflected by increased mRNA transcription of the fatty acid binding protein. Prolonged low-energy diet promoted cell-specific heat shock protein (Hsp) response; whereas significant increase of Hsp90 but unchanged levels of Hsp70 proteins were observed in white blood cells, the expression of Hsp70 in milk somatic cells was markedly attenuated, in parallel with a marked increase of αs1-casein expression. At the mammary gland level, these results may indicate a decrease in turnover of proteins and a shift to an exclusive expression of milk components producing factors. Similar responses to caloric stress were revealed also in ketotic dairy cows. Ketosis promoted a shift towards long and unsaturated fatty acids and an increased expression of αs1-casein in milk somatic cells. These findings may reflect an evolutionary-preserved mechanism in lactating cows for coping with caloric restriction. Overall, our results provide an index to test suitability of beef cattle breeds to inadequate caloric demands. © Cell Stress Society International 2008.