Exposure to 41 degrees C reduces development of embryos of heat-sensitive breeds (Holstein and Angus) more than for embryos of the heat-tolerant Brahman breed. Here it was tested whether embryonic resistance to heat shock occurs for a thermotolerant breed of different genetic origin than the Brahman. In particular, the thermal sensitivity of in vitro produced embryos of the Romosinuano, a Bos taurus, Criollo-derived breed, was compared to that for in vitro produced Brahman and Angus embryos. At d 4 after insemination, embryos > or = 8 cells were randomly assigned to control (38.5 degrees C) or heat shock (41 degrees C for 6 h) treatments. Heat shock reduced the proportion of embryos that developed to the blastocyst stage on d 8 after insemination. At 38.5 degrees C, there were no significant differences in development between breeds. Among embryos exposed to 41 degrees C, however, development was lower for Angus embryos than for Brahman and Romosinuano embryos. Furthermore, an Angus vs. (Brahman + Romosinuano) x temperature interaction occurred because heat shock reduced development more in Angus (30.3 +/- 4.6% at 38.5 degrees C vs. 4.9 +/- 4.6% at 41 degrees C) than in Brahman (25.1 +/- 4.6% vs. 13.6 +/- 4.6%) and Romosinuano (28.3 +/- 4.1% vs. 17.5 +/- 4.1%). Results demonstrate that embryos from Brahman and Romosinuano breeds are more resistant to elevated temperature than embryos from Angus. Thus, the process of adaptation of Brahman and Romosinuano breeds to hot environments resulted in both cases in selection of genes controlling thermotolerance at the cellular level.
Beef cattle breeding has developed in extensive grazing systems in Mexico, concerning the livelihood of over one million families and affecting the use of natural resources. Reproductive efficiency is a major determinant of both the environmental impact of the herd, and the economic viability of the cow-calf production system. As reproductive traits have low heritability, reproductive efficiency can be largely influenced by herd management practices. Thus, a questionnaire was administered as personal interviews to 3280 producers, to investigate the prevalence of practices that could directly influence the reproductive outcome of their cattle. Results show that yearround breeding (93%) and natural service by the bull (97.4%) are the most common mating methods. However, only 41% of producers evaluate the breeding soundness of their bulls, and diagnostic testing for specific reproductive diseases of the sires is barely performed in 20% of the farms. The main declared reason for replacing the bull is old age (26.8%), which is followed by to avoid inbreeding (68.4%). Less than 10% of the operations use methods for the synchronization of the estrous cycle, and most farmers breed their heifers for the first time after 2 years of age (73%). Diagnosis of pregnancy is performed in merely 31% of operations, with 23% of the producers declaring to discard non-pregnant cows. Energy or protein feed supplements are provided to the cattle in 63% of the production units, whilst mineral supplements are given in 79% of the farms. Calves are typically weaned between 6 months and a year of age (85%). Only 16% of operations vaccinate against brucellosis and 17.5% against other reproductive
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