Effect of nutrition on follicle development and ovulation rate in the ewe

The general aim of this thesis was to gain further knowledge on the effect of nutrition on endocrine response and follicular dynamics in the cyclic ewe. Paper I presents an investigation of the effect of low and high body condition on follicular dynamics and endocrine profiles in ewes. The study showed that ewes in high body condition had a constant pattern of three follicle waves, while ewes in low body condition developed two or three waves during the cycle. Individual follicle tracking by ultrasonography (US) made it possible to study the growing profile of each follicle in relation to its endocrine milieu. Oestradiol and FSH levels were implicated in the regulation of follicle turnover. Ewes in a high body condition had a higher ovulation rate, which was associated with higher FSH and lower oestradiol concentrations during the follicular phase of the cycle. There was a lack of information regarding the limitations of US to evaluate follicles in different size and ovulation rate, which could replace more invasive techniques. The objective of Paper II was to assess the accuracy of ultrasonography in evaluating the number of follicles and corpora lutea (CL) compared with observations made at post-mortem examination of the ovaries. The predictive value and sensitivity of US was 100% for the presence and 96% for the absence of CL. The sensitivity was 80% for the determination of two CL. The sensitivity was high (90-95%) for follicles ³4 mm. The lower sensitivity (62%) and predictive value (71%) of US for evaluating smaller follicle classes (2 and 3 mm, respectively) may explain inconsistent results found in the literature. Overall, the study demonstrated that US is a good tool for counting the number of CL to evaluate ovulation rate under field conditions. In Paper III we evaluated if a short-term nutritional supplementation before ovulatory wave emergence could increase the number of follicles recruited into this wave, in ewes in low body condition. The supplement increased the proportion of ewes with a three-wave pattern, but neither FSH concentrations nor follicle recruitment were increased. In the short-term supplemented group the higher feed intake induced a decrease in progesterone concentrations, which prolonged the life span of the largest follicle present at the beginning of the nutritional treatment. A field trail was performed to evaluate the effect of the same nutritional treatment on ovulation rate. Ewes in moderate-high body condition had a higher ovulation rate than ewes in low body condition. In conclusion, short-term supplementation of ewes prolonged the life span of the last non-ovulatory follicle and promoted a 3-wave pattern of follicle development during the inter-ovulatory interval. Supplementation also promoted the development of a more active dominant follicle in ewes with a single ovulation. Short-term supplementation from Day 8 to Day 14 of the oestrous cycle increased ovulation rate in ewes with a moderate to high body condition. In Paper IV the hypothesis tested was that glucose, insulin, leptin and IGF-I are increased during a short-term supplementation. Acute changes in the peripheral concentrations of glucose and metabolic hormones related to time of feeding were measured. The results showed that glucose, insulin and leptin increased in the supplemented group on the third day after supplementation commenced. IGF-I concentrations were similar among groups. Thus, the effect of a short-term nutritional supplementation on ovulation rate in the ewe may be mediated through an increase in the concentrations of glucose, insulin and leptin.