7CHAUHAN I S, GUPTA A K, KHATE K, et al. Geneticand non genetic factors affecting semen production traits in Karan Fries crossbred bulls[J]. Tropical Animinal Health and Production ,2010,42 = 1809-1815.
8HELBIG L,WOODBURY M R, HAIGH J C, et al. The seasonal fertility of north American bison (Bison bison ) bullsEJ]. Animal Reproduction Science, 2007,97 : 265-2?7.
9ERB R E,ANDREWS F N, HILTON J H. Seasonal Varia tion in Semen Quality of the Dairy Bull~J~. Journal of Dairy Science, 2013,91 : 5821 5828.
10BHAKAT M,MOHANTY T K,RAINA V S,etal. Effect of age and season on semen quality parameters in Sahiwal bulls [J]. Tropical Animinal Health and Production, 2011,43:1161-1168.