Maturational changes during the follicular phase of the porcine oestrous cycle were investigated in spontaneously cycling gilts. The results revealed considerable variation in the morphological and biochemical development of dominant follicles in the early follicular phase. This asynchrony was emphasised by the marked differences in follicular fluid steroid concentrations and LH binding to granulose cells and theca tissue between follicles of identical size within the same ovary. The nature of this variation may have great significance on our present understanding of asynchronies in embryo development.
On the basis of the variables assessed in this study, those follicles destined to ovulate or to become atretic were only readily distinguishable on Day 20 of the oestrous cycle, suggesting that the selection of the preovulatory population continues over a 4 day period from about Day 16 to Day 20 of the cycle and, furthermore, that the selection process involves some initial maturation even in those unselected follicles that ultimately join the atretic pool.
Leading on from this study, the control of the initiation of follicular development in lactating primiparous sows was examined by assessing the relationships between ovarian activity and circulating hormone levels following manipulation of the suckling stimulus.
In the first experiment, piglet suckling behaviour and follicular development were 'investigated following a reduction in litter size to five piglets (split-weaning), seven days before weaning at 21 days post-partum. Overall, morphological and biochemical follicular development were significantly advanced (p < 0.001) in the treatment sows. The significantly (p < 0.001) greater concentration of follicular fluid oestradiol in follicles recovered from the split-weaned sows suggested that the supply of androgen substrate was probably limiting production of this steroid in the control animals. It was proposed that the latter was due to inadequate gonadotrophin stimulation.
The comparative ethological study revealed that the remaining split-weaned piglets 'multiple-suckled' quarters vacated by their heavier litter mates. Despite a constant suckling frequency, suckling intensity was reduced in the split-weaned litters as evidenced by a regression in mammary tissue growth in the dams.
In a subsequent experiment, further manipulation of the suckling stimulus was achieved by physically restricting the number of mammary quarters available to the piglets. Canvas sheeting strapped around the anterior quarters of one group of sows (cover) limited the five piglets remaining after split-weaning to suckle only the posterior quarters. An investigation of circulating hormone levels, 12 hours before and 48 hours after the start of treatment, revealed that, compared to the split-weaned and control sows, LH levels were significantly (p < 0.001) elevated in the cover sows with a concomitant significant (p < 0.001) depression in circulating prolactin. Although split-weaning induced a similar endocrine response in several sows, this effect was short-lived and overall, levels of both hormones were similar in the split-weaned group to those recorded in the control sows. The concentration of oxytocin, released at suckling was unaffected by litter size or the intensity of mammary stimulation.
Ovarian activity at weaning was significantly enhanced in the cover (p < 0.001) and split-weaned (p < 0.001) groups. Hence, it was concluded that the critical factor influencing hormone levels and follicular development in the lactating sow was the neural intensity of the suckling stimulus.
The cumulative data of the lactation experiments suggest that in practice, serious consideration should be given to the use of litter size reduction as a means of effectively improving the performance of the primiparous sow and her litter during lactation
