Causes and consequences of variation in weaning to oestrus interval in the sow = Oorzaken en gevolgen van verschillen in interval spenen - bronst van zeugen

First parity sows often have a prolonged weaning to oestrus interval (WOI), are treated with oestrus inducing hormones or are not showing oestrus at all. Many young sows are culled because of fertility problems. Of the annual culling of sows, 25 to 30% is because the sow is not showing oestrus or not pregnant. The WOl in current pig husbandry is, with an average of 7 days or less, rather short. The research into causes and consequences of a prolonged WOI was started because of possible consequences of even little variation in WOl for sow reproductive performance. Later research was focussed on mechanisms explaining consequences of a variation in WOI. Finally some possibilities to improve sow reproductive performance were investigated.The overall objective of this study was to determine the causes of variation in sow production traits as associated with differences in WOl and to develop strategies to improve sow production.An observational study was conducted to determine the consequences of variation in WOl for sow production (Chapter 2). An increase in WOI from 4 to 6 days went with a decrease in litter size (0.4 piglet) and a decrease in farrowing rate (5% in higher parity sows and 9% in first parity sows with an increase from 5 to 6 days). With a further increasing WOI, litter size decreased further. Sows with a WOI of 9-12 days produced about 0.8 piglets less in their next litter. The farrowing rate of these sows was also 29% lower than sows with a WOl of 5 days. Sows with a further prolonged WOl produced better. Sows with a WOIof 13-18 days produced a number of live born piglets, comparable with that of sows with a WOl of 5 days. Sows with a WOI of 19 days or more, clearly produced more piglets in their next litter compared with sows with a shorter WOI; 1.8 piglets more than sows with a WOl of 5 days and 2.5 piglets more than sows with a WOI of 9-12 days.The outcome of this study was, that variation in WOI is associated with clear differences in sow production traits. Based on this first observational study, research into the causes of the depressed reproduction results that are associated with a small increase in W01, was indicated. The question to be answered was, whether there are possibilities to prevent the depressed production associated with a slightly prolonged WOI.Because of the negative effects on sow production of a further prolongation of the W0I, It was also indicated to investigate how a prolonged W0I can be prevented. Research into the causes of a prolonged W0I in current pig husbandry, therefore, was indicated.Based on the results of this first study it also was indicated to look for management methods to achieve an increase in litter size as found for sows with a very long W0I.An experiment was designed with sows in oestrus between 4 and 6 days after weaning to determine the causes of the consequences of little variation in W01 in terms of litter size and farrowing rate (Chapter 3). During oestrus two inseminations were performed on a fixed time during the day (around 14.00 h), with an interval between subsequent inseminations of 24 h. The first insemination was performed on the first day the sow was showing standing heat. Boars of two different breeds were used for subsequent inseminations within one oestrus, to make it possible to classify the offspring after the boar used for first or second insemination. The results showed that sows with a W01 of 4 days had less offspring from first insemination and more from the second. With an increasing W01 more offspring from the first insemination was born and less from the second. In this experiment, litter size itself was not influenced by W01. The distribution after litter type - completely from first insemination, mixed or completely from second insemination - showed comparable results. Sows with a W01 of 4 days had less litters completely from first insemination, an equal percentage mixed litters and more litters completely from second insemination compared with sows with an W01 of 5 days. An explanation for the differences found is, that sows with a longer W01 ovulate sooner relative to the insemination moment (and relative to the start of the oestrus, when checked as described).With the results of this experiment the lower farrowing rate and smaller litter size in sows with a somewhat prolonged WOI (6 days) may be explained. Sows with a somewhat longer WOI should, based on these results, be inseminated a little sooner to increase their production.An effect of genotype of the sow and parity, on the number of offspring from each off the two inseminations, was also found in this experiment. Finnish Landrace and Great Yorkshire sows should be inseminated sooner than Dutch Landrace sows and sows of parity 6 or higher should be inseminated sooner than lower parity sows.A second observational study was performed to determine the causes of prolongation of the WOI (Chapter 4). This study showed an increase in WOI and use of oestrus inducing hormones (PG600 r) in first and second parity sows increased with an increasing body weight loss during the preceding lactation. In third and highe parity sows no effect of increase in body weight loss during lactation on WOI or use of PG600P was found. The housing system in which sows were housed during gestation (from one month after service until one week before farrowing) also showed to be of influence on the WOI and use of PG600 r. The WOI of first and second parity sows in the group housing system was longer than that of first and second parity sows in two individual housing systems and the use of PG600 rwas higher. The effect of body weight loss during lactation and housing system was greater in first than in second parity sows. Other factors showed, independent from parity, a difference in length of the WOI and use of PG600 r, but this difference was smaller than the differences found for parity and housing system. These other factors were: breed, number of piglets weaned and season.Based on the results of the observational studies, the second and third experiment were designed (Chapter 5 and 6). The objective was to influence the WOI and/or subsequent reproductive performance in a positive way. These experiments were performed with young sows, because in these animals the largest effect can be expected. It seems very promising to influence the body weight loss during lactation. The restoration of body weight after weaning by giving extra time between weaning and first insemination, seems a good alternative. Also because the litter of sows with a very long W0I was so much larger.In the second experiment it was investigated whether the weight loss during lactation of first and second parity sows could be reduced by reduction of the litter size to 6 piglets, during the last week of a four week lactation period (split-weaning; Chapter 5). Both factors weight loss during lactation and litter size at weaning, are of influence on the length of the W0I (Chapter 4) and may be influenced at the same time by the lay out of this experiment. An effect of split-weaning on body weight loss, but also on backfat loss, during lactation was effected in the second parity sows only. In the first parity sows the differences in weight loss and backfat loss during lactation were not significant. A significant effect of split-weaning on W0I was found in second parity sows only, a reduction in W01 of 0.8 day. The distribution of sows after W0I changed by the treatment: more sows with a W01 of 4 days, an equal part with a W0I of 5 days and less sows with a W0I of 6 days or more. Consequences for subsequent reproduction were also only found in second parity sows: the FR in the split-weaned second parity sows increased from 86% to 97%. In first parity sows a more severe reduction in litter size, or a longer period of reduction in litter size, may be necessary to influence the weight loss during lactation and through this the W0I and subsequent production.A third experiment was designed with the purpose to investigate whether a prolongation of the weaning-to-insemination interval to over 19 days, improves the litter size. By skipping the first oestrus after weaning in first parity sows to breed during the second, the weaning-to-insemination interval was prolonged with 21 days (the length of an oestrous cycle) to 27 days. Compared with breeding during first oestrus, this resulted in a increase in litter size of 1.2 live born piglets (from 10.6 to 11.8 piglets; Chapter 6) and an increase in farrowing rate of 15%. In this experiment, indications were found for a possible long term effect of this treatment. The higher farrowing rate and a larger litter size of the third litter of sows bred at second oestrus after first weaning, however, was not significant.In the general discussion (Chapter 7) the results found in this thesis are discussed from possible underlying physiological processes.According to the literature a prolonged WOI is associated with a shorter duration of the oestrus and an ovulation moment earlier after the start of oestrus. It is also described that as long as the interval between insemination and ovulation is between 24 and 0 h, no effect was found on number and quality of the embryo's. A bad synchronisation between insemination moment and ovulation moment, associated with small increase in W0I, may cause a reduced percentage fertilized eggs or no fertilization at all. This, whereas the fertility of the sows is normal. The reduction in litter size and farrowing rate of sows with a slightly increased W01 (from 4 to 6 days) seems to be the consequence.Variation in farrowing rate and litter size may also be explained as the consequence of the metabolic state of the sow in during the period preceding ovulation. A catabolic state during follicular growth is unfavourable for the quality of follicles andlor eggs and thus for ovulation rate and embryo survival. The smaller litter size and lower farrowing rate of sows with a W0I of 9-12 days, may be explained this way; a very catabolic state of the sow leads to both a prolongation of the W0I and a less favourable follicular development, which already had started in the period the sow was catabolic. The larger litter size and higher farrowing rate in sows with a W0I of 19 days or more and the smaller litter size and lower farrowing rate of sows with a W0I of 9-12 days may also be explained from the metabolic state during follicular development. Follicular development in sows with a very prolonged W0I takes place during the period after weaning and, therefore, under favourable circumstances. This may result in a better follicle and egg quality and this may explain the higher farrowing rate and larger litter.Season itself was hardly of influence on the W0I, but high ambient temperatures during the summer may reduce the feed intake during lactation and thus the metabolic state may be influenced. So season may, indirectly, have important effects on the W0I and litter size and farrowing rate. The depressed reproductive performance of sows in late summer and autumn, after a hot summer period, may be explained this way.Group housing (with a large dynamic group of sequential fed sows) was associated with a prolonged W01 in young sows. This effect may be explained by an effect of social rank during gestation on oestrus expression after weaning. Effects of social rank on oestrus expression, depressed display of oestrus or no oestrus at all, are also known in other species. Sows low in social rank and therefore showing their oestrus somewhat later, may subsequently be inseminated too late (sows with a W01 of 6 days). In an extra analysis, the lower farrowing rate found in first parity sows with a W01 of 6 days found in Chapter 2, turned out to be caused for a great part by the first parity sows from the group housing system and for a smaller part by sows from the two individual housing systems.Main conclusions.The length of the W0I has a pronounced effect on subsequent farrowing rate and litter size. With an increasing WOI up to 12 days the farrowing rate and litter size are decreasing. With a further increasing W0I, however, litter size is increasing again and sows with a WOI of 19 days or more even produce 1.8 piglets per litter more than sows with a WOI of 4 to 5 days.The reduction in litter size with a small increase in W0I, a WOI between 4 and 8 days, may be explained by a poor synchronisation of insemination and ovulation moment. This effect on litter size may be prevented by an adjustment of the insemination strategy: inseminating sows that show oestrus later after weaning, sooner during that oestrus.First and second parity sows showe an increase in W0I when the weight loss during lactation is increasing. Housing gestating sows in large, instable, sequential fed groups, also causes a prolongation of the W0I of first and second parity sows. The number of piglets nursed, breed or crossing of the sow and season are also of influence on the length of the W0I, but the effect of these factors is smaller.Split-weaning of first and second parity sows to improve both W0I, subsequent litter size and farrowing rate, has been found only beneficial in second parity sows. The second parity sows show a reduction of the W0I and an increase in farrowing rate of 11 %. The effect of this treatment is not enough to influence W0I or subse quent farrowing rate and litter size of first parity sows.By skipping the first oestrus after weaning in first parity sows, the litter size can be improved with 1.2 live born piglet and the farrowing rate with 15%. Furthermore, there are indications that the next farrowing rate and the size of the third litter is certainly not worse compared with that of sows in which the first oestrus is not skipped

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