Seasonal Variation in the Reproductive Hormones of Rabbit Does and its Relation to Their Reproductive Performance

The present study was carried out at the Rabbits Experimental Farm, belonging to the Department of Animal Production, Faculty of Agriculture, Al–Azhar University. Twenty Californian does aged 3 months with average live body weight of 2.2kg were used (10 in each season). Rabbit that born in February (ten does) represented the summer group while that born in August represented the winter group. Breeding does and bucks were housed individually in wire bottomed-cages in visual contact with their neighbors with standard dimension arranged in double-tier batteries of column type. Mash ration of about 16% total protein and 60% starch equivalent was provided ad lipitum twice daily in the morning and at evening. During winter barseem (Egyptian clover) Trifolium alexandrinum was provided. During summer months clover hay was supplied. Fresh clean water was available to rabbit all times. Weaning was practiced at the 21th day after parturition. The following reproductive and productive traits were estimated in each season: Age and live body weight at puberty, litter size at birth, mortality rate and litter interval. Serum estradiol-17 and progesterone levels were measured at the following physiological stages: Pre-puberty, at puberty, during gestation period (1st, 2nd and 3rd weeks of gestation) and during lactation period (1st, 2nd and 3rd weeks of lactation). The objective is to study the seasonal variation in the levels of female sexual steroidal hormones (estradiol 17 and Progesterone) with respect to the productive and reproductive activity in does of California breed.

Results indicated that in winter, serum level of estrogen was significantly (p≤0.05) higher at puberty than pre-puberty, pregnancy and lactation levels except at the third week of lactation. During pregnancy, estrogen level tended to increase during the second week of pregnancy until the end of pregnancy period (at parturition). After lambing, serum estrogen tended to increase linearly with the advance in lactation although the changes were not statistically significant. A similar trend was found in summer, however the increase in estrogen level during mid-pregnancy (second week) as well as during the end of lactation period (third week) was significant (p≤0.05). In both seasons, serum progesterone level did not differ significantly between at puberty and pre-pubertal stages, but increased dramatically to reach its highest level at early pregnancy. During summer season serum progesterone level decreased during mid-pregnancy and slightly increased towards the end of pregnancy. In contrast serum progesterone level decreased towards the end of pregnancy period in winter season. After delivery serum progesterone gradually decreased during lactation period reaching almost the pre-pubertal level at the end of lactation period. Regarding the effect of season, serum estrogen concentration during different physiological status was higher under heat stress in summer than under thermoneutral temperature in winter. The difference was significant at the pre-pubertal stage and during different stages of pregnancy. An opposite significant trend was found in serum progesterone levels which were significantly higher in winter than in summer during pregnancy and early lactation. Reproductive performance was better in winter than in summer where does bred in winter season had significantly shorter litter intervals than had those bred in summer season. Also, letter size was slightly higher in winter with significantly higher survivability (number of live bunnies). It could be concluded that higher serum progesterone and lower serum estrogen during pregnancy in winter than in summer may be related to better reproductive performance in winter than in summer and breeding season must be taken in consideration if high efficiency of reproduction is aimed to be obtained.