Effect of milk- and TRIS-based extenders on the fertility of sheep inseminated vaginally once or twice with liquid semen

We studied the influence of two different extenders, a milk-based versus a TRIS-based extender, using a split-sample technique, on fertility after single and double vaginal inseminations in natural estrous in Norwegian Crossbred ewes. Semen from 21 Norwegian Crossbred rams, all aged approximately 0.5 years, was used for AI of totally 561 Norwegian Crossbred ewes housed at 37 different farms. The farmers performed the inseminations themselves. The ewes were allocated to four parallel groups based on the two extenders and single or double inseminations (2×2). The farmers were recommended to inseminate the ewes between 12 and 24 h after detection of natural standing estrous. Vaginal insemination with cooled liquid semen diluted in the milk-based extender resulted in a statistically significant ( P