Induced seasonal reproductive performance in two breeds of sheep

Genomic Approaches Identify Novel Gene Associations with Out of Season Lambing in Sheep

Sheep are seasonally polyestrous, traditionally breeding when the day length shortens in the autumn. The changing photoperiod stimulates reproductive hormones through a series of chemical pathways, ultimately leading to cyclicity. Some breeds of sheep, such as the Polypay and Dorset, have been selected for reduced seasonality and can lamb year-round. Despite this selection, there is still variation within these breeds in the ability to lamb out of season. The identification of out of season lambing quantitative trait loci has the potential to improve genetic progress using genomic selection schemes. Association studies, fixation index (FST), and runs of homozygosity (ROH) were evaluated to identify regions of the genome that influence the ability of ewes to lamb out of season. All analyses used genotypic data from the Illumina Ovine HD beadchip. Genome-wide associations were tested both across breeds in 257 ewes and within the Dorset and Polypay breeds. FST was measured across breeds and between UK and US Dorsets to assess population differences. ROH were estimated in ewes to identify homozygous regions contributing to out of season lambing. Significant associations after multiple testing correction were found through these approaches, leading to the identification of several candidate genes for further study. Genes involved with eye development, reproductive hormones, and neuronal changes were identified as the most promising for influencing the ewe’s ability to lamb year-round. These candidate genes could be advantageous for selection for improved year-round lamb production and provide better insight into the complex regulation of seasonal reproduction.

Plasma progesterone concentrations during early pregnancy in spring- and autumn-bred ewes

The objective of this experiment was to measure blood progesterone concentrations during early gestation to determine if the apparent reproductive failure in ewes bred out-of-season is due to a failure to conceive or embryonic loss. Blood samples were collected from spring- (n=61) and autumn-bred ewes (n=29) from Days 8 to 39 post-oestrus. Serum progesterone concentrations were analysed to ascertain whether ewes were ovulating and failing to maintain pregnancy, or conception was failing. Following pregnancy diagnosis 62 days after ram introduction, ewes were categorised as; no display of oestrus, mated but then identified as non-pregnant, or pregnant. A majority of spring-bred ewes that failed to display oestrus had silent oestrus (86%) and 66% of those ewes had abnormally short-lived corpora lutea. Circulating progesterone concentrations during dioestrus in ewes that had ovulated and displayed oestrus were unaffected by season. Similarly, progesterone concentrations during dioestrus did not differ between pregnant and mated non-pregnant ewes. The results indicated that while early luteylosis, low progesterone secretion from corpora lutea and embryo mortality did occur, these were in only a small proportion of ewes. Progesterone concentrations indicated that a majority of mated non-pregnant ewes had elevated progesterone concentrations necessary for the production of at least one viable embryo/foetus. This may be indicative to the failure of maternal recognition of pregnancy, and it is recommended that events surrounding this stage of pregnancy (Days 12-14) be examined more closely in ewes during the non-breeding season.

Melatonin-improved reproductive performance in sheep bred out of season

The effects of melatonin implants on out-of-season breeding in New Zealand Romney composite ewes, was determined by comparison of reproductive performance in ewes treated with progesterone+equine chorionic gonadotrophin (eCG) (control; n=107), melatonin+progesterone+eCG (n=97) or melatonin+progesterone (n=96). Conception rates in melatonin+progesterone+eCG-treated ewes (67%) were higher than in the control ewes (P<0.01; 47%). Pregnancy rates were higher in melatonin+progesterone+eCG-treated ewes (55%; P<0.001) compared with the control ewes (40%). Fewer melatonin+progesterone-treated ewes displayed oestrus (14%; P<0.001) and subsequently became pregnant (6%). Oestrus rates in melatonin+progesterone-treated ewes (14%) were lower than both the melatonin+progesterone+eCG-treated (82%) and control ewes (86%; P<0.001), which were similar to each other. The number of foetuses per pregnant ewe was similar in all three treatment groups. Serum melatonin concentrations at Day -9 were higher in the ewes treated with melatonin and there was a large variation between individual ewes, but concentrations were similar for pregnant and nonpregnant ewes. The combination of higher conception rate and the trend for more lambs per pregnant ewes resulted in more lambs being born per ewe treated in melatonin+progesterone+eCG-treated ewes compared to the other two treatment groups. These results suggest that melatonin implants, in conjunction with administration of progesterone and eCG, may be suitable as a means of increasing the number of lambs born per ewe treated in an out-of-season breeding program in New Zealand sheep flocks while melatonin and progesterone is not.