high yielding breeds of cattle have been showing dramatic decrease in fertility rates since mid-1980s, despite significant improvement in dairy-management technologies. A new study has shown this is largely due to physiological imbalances caused by seasonal factors. High-yielding varieties are being widely promoted in India.

The study says reproduction problems become acute in high summer temperature and peak winter when the animal's body temperature does not match with the ambient temperature. This along with sunlight energy, humidity and wind speed, causes heat stress. "High-yielding breeds are usually adapted to some agro-climatic conditions. When these conditions change, the productivity declines," says Gurmej Singh of National Bureau of Animal Genetic Resources, Delhi.

Heat stress is said to cause ovulation failure and affect female germ cell, sperm cell and embryos.Ovulation failure has been found to be about four times higher in cows inseminated in a warm period (May-September). High temperature on a day after insemination has also found to reduce conception rate, says the paper published in the journal Theriogenology (Vol 67, No 8).

The paper says keeping the maximum daily temperature at 20