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  • Title: Is the yak (Poephagus grunniens L.) really a seasonal breeder?
    Author: Sarkar M, Meyer HH, Prakash BS.
    Journal: Theriogenology; 2006 Mar 01; 65(4):721-30. PubMed ID: 16084578.
    Abstract:
    Yaks are considered to be seasonally polyestrous and breeding occurs from July to November. Here we show that some yaks in peak non-breeding season do exhibit cyclic luteal activity without exhibiting any behavioral signs around expected estrus. A total of eight non-lactating yaks were selected from the Yak Farm belonging to National Research Centre on Yak for various sets of experiments. The animals were maintained as per semi range system of management. They were allowed to graze during daytime and fed concentrate mixture @2 kg/animal/day as per standard farm practices of the center. Blood samples were collected on alternate days for 30 days by jugular venipuncture from the yaks during peak breeding season (July to November) and from the same yaks in non-breeding season (February to March). The plasma samples were analysed for progesterone and estradiol-17beta by RIA and EIA procedures, respectively. During breeding season, the mean plasma progesterone at estrus was basal (<or=0.2 ng/ml) and there after started to increase and reached a peak at days 15-16 of the cycle and then declined rapidly to basal levels at estrus. Mean plasma estradiol-17beta concentrations were high at estrus and then declined to basal level at days 2-3 of the cycle. The mean plasma estradiol-17beta concentrations stayed low for the remaining part of the estrous cycle except for a small elevation, which was exhibited between days 6 and 12 of the cycle. During non-breeding season, in 50% of the animal studied, plasma progesterone and estradiol-17beta levels were at basal level. However, in the remaining animals distinct luteal activity was recorded in terms of plasma progesterone profile. High concentrations of progesterone were maintained in one animal, which suggested existence of cystic corpus luteum. The trend of progesterone and estradiol-17beta profiles in other three animals indicated that they could be cycling although they did not exhibit estrus symptoms. It was concluded from the present study that with improved nutritional support some yaks exhibit estrous cyclicity even during the non-breeding period of the year.
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