Year: 2008
Type: dissertation
Abstract: Replacement gilts and early parity sows constitute a large, and increasing, \nproportion of modern breeding herds. Breeding herd profitability is therefore \nincreasingly dependant on the efficiency of gilt management strategies as well \nas litter size at first farrowing; however, incidences of reproductive failures and \nsmall first litter sizes are common within cohorts of replacement gilts. Hence, \nthis thesis had two primary aims which were addressed in four experiments; \none, to identify whether the puberty stimulation and mating strategies \ndeveloped for genotypes of 20 to 30 years ago are suitable for today’s heavier \nyet leaner genotypes; and two, to better understand the influence of pre-pubertal \ngrowth rate and metabolic status on reproductive maturation, puberty \nattainment and potential litter size. \nIn experiment 1, 192 Large White/Landrace crossbred gilts were used to \ncompare the effects on puberty attainment of commencing boar exposure at \n161, 182 or 203 days of age, and the effect of first mating gilts at either the \npubertal or second oestrus on ovulation rate and early embryo survival. Gilts \nwere artificially inseminated at the allocated oestrus, with the reproductive \ntracts collected at 22.8 ± 0.4 days after first mating (mean ± S.E.M), and the \nnumbers of corpora lutea and viable embryos recorded. Delaying first boar \ncontact until 182 or 203 days of age significantly (P < 0.01) reduced days-to puberty \nand increased the proportion of gilts attaining puberty within 10 days of \nstart of boar exposure. Gilt age at mating had no effect on potential litter size; \nhowever, there was a tendency for gilts mated at their second oestrus to shed \n0.6 more ova, and possess one more embryo at day 20 of pregnancy. \nExperiment 2 determined the effects of long- (chronic) and short- (acute) term \nmoderate dietary restriction on ovarian development and oocyte developmental \ncompetence in 161- and 175- day old, pre-pubertal gilts. Both chronic and acute \nperiods of moderate feed restriction reduced the number of medium follicles \npresent on the ovaries of 161- and 175-day old gilts, as well as the proportion of oocytes reaching Metaphase II in vitro. However, feeding level during the 14 \ndays prior to ovary collection had the greatest effect on follicular growth and \noocyte quality. Experiments 3 and 4 investigated the effects of the same dietary \ntreatments on the timing of puberty attainment in response to boar exposure \nand potential litter size following mating at the pubertal oestrus. Chronic dietary \nrestriction during the pre-pubertal period delayed puberty attainment, but the \ntiming of the pubertal response was unaffected by acute, moderate dietary \nrestriction of previously well-fed gilts during the period immediately prior to, \nand coincident with, boar exposure. Acute dietary restriction or repletion \nstimulated an increase or decrease, respectively, in pubertal ovulation rates; \nhowever, neither the number of viable embryo present on day 22 of gestation \nnor embryo survival were affected by the nutritional treatments used in these \nstudies. \nOverall, these results demonstrate that the timing and synchrony of puberty \nattainment is significantly improved when gilts first receive boar exposure at \n182 days of age (or older). It is, therefore, concluded that sexual maturity, as \nmeasured by responsiveness to boar contact, occurs later in modern \ngenotypes. It is also evident that within the age range investigated, delaying \nfirst mating until the second oestrus does not significantly increase either \novulation rate or embryo number at day 20 post-mating. Further, the current \ndata provide the first evidence that despite profoundly affecting the size and \nmorphology of the antral follicle pool as well as pubertal ovulation rates, subtle \nalterations in dietary intake have no affect on the number or proportion of \nembryos surviving the pre-and peri-implantation period. It is evident the litter \nsize of gilts mated at the pubertal oestrus is not determined by the number of \nova shed, with the current data demonstrating that increasing ovulation rates \nresults in increased embryo mortality.
Author: William van Wettere
Language: English
Subfield: Animal Science and Zoology
Domain: Life Sciences
SDG: Zero hunger
Open Access status: green
OpenAlex ID: W2270026629