null 33. HURA,V., HAJURKA,J., KACMÁRIK,J., CSICSAI,G., MACÁK,V., VALOCKÝ,I.: The effect of some factors on gestation length in the Nonius breed mares in Slovakia. (Egyes tényezõk hatása a nóniusz fajta vemhességének idõtartamára)

33. HURA,V., HAJURKA,J., KACMÁRIK,J., CSICSAI,G., MACÁK,V., VALOCKÝ,I.: The effect of some factors on gestation length in the Nonius breed mares in Slovakia. (Egyes tényezõk hatása a nóniusz fajta vemhességének idõtartamára)

33. HURA,V., HAJURKA,J., KACMÁRIK,J., CSICSAI,G., MACÁK,V., VALOCKÝ,I.: The effect of some factors on gestation length in the Nonius breed mares in Slovakia. (Egyes tényezõk hatása a nóniusz fajta vemhességének idõtartamára)

33. Hura

érk: 97.10.16.



UVL, 04101 Kosice, Komenského 73, Slovak Republic


SUMMARY


Some factors were examined and their effect on gestation length. Gestation length of 1575 mares terminated by bringing one or more normally developed foals was studied. The average length was 332,37 days with fenotypical standard deviation 6,9 days. The range was 297 - 414 days. The age of the mare the number of foaling in life of the mare did not effect the length significantly. The fillies had gestation length 3,43 days (P<0.05) and twins 7,11 days shorter than colts (P<0.01). The effect of the year was significant (P<0.01). Differences between years were up to 9,5 days. The most signifi-cant proved to be the effect of the season - month. Mares bred in months January - May had aver-age gestation length 10 days longer than mares bred in July - October (P<0.01). The effect of the date of parturition is similar. Mares foaling in July - October had average gestation length 13,6 days shorter than January - April parturitions (P<0.001).
The explanation requires a few more questions to solve why some factors have such significant effect on gestation length, like e.g. photolength and photoreception, uterinal distention and the sex of the fetus in relation to hormonal processes which start the parturition.


INTRODUCTION


One of the most frequently examined questions in the field of equine reproduction is gestation length in mares. The real gestation length should be calculated from the day of fertilization (ovulation) to the date of parturition. However, the date of parturition cannot be determined exactly, it is calculated with the date of the last breeding of the mare. Regarding the fact that mares are bred several times during foaling heat, there are differ-ences in determinations of gestation length. It can be as long as a week. The exact determination of gestation length can be very useful and the knowledge of the difference of gestation length and factors effecting it can save trouble to the owners of mares. The average gestation length is 333 days, but some authors give range 335 - 340 days, with ever bigger deviations. Pribyl (1952 cit. 5), considers the interval 307, resp. 412 days as extreme. The purpose of our study was to assess the effect of the age, the number of parturition, sex, breeding season, the year and the month of foaling on gestation length and according to the result to forecast the date of foaling as exactly as possible.


MATERIALS AND METHODOLOGY


We performed the study at a Thoroughbred farm of Nonius breed at the stud farm Novy Tekov in Slovakia. In the period 1965 - 1996 we recorded 1575 gestation lengths of 298 mares by 32 different stallions. The effect of age, the number of gestation, sex, breeding season, month of breeding and parturition was examined. Gestation length was calcula-ted from the date of the last copulation until parturition recorded in the proper books of breeding. The observed data were compared and statisticaly assessed by Student's T-test for pair and impair values. (Student T-Test).


RESULTS AND DISCUSSION


The average gestation length was 332,37 days with the standard deviation 6,9 days. The range was 297 - 414 days. The average gestation length of twins was 326,86 days with the standard deviation 10,2 days with the range of 302 - 386 days.
Although some preceeding reports indicate that older mares can be pregnant longer time than the young ones 1,2,4. Other reports confirm that the age of the mares does not effect the length or that the young ones have longer periods 3.11.14. The study (Tabl. 1) discovered that the age does not effect gestation length which is confirmed by the fact that gestation length is not related to its sequence number in life of the mother (Tabl. 2). However we state that in the study the young, the first time foaling 3,4 years old mares had in average the longest gestation in comparison from 1 st to 10th parturi-tion in the same mare. Controversions of the reports as well as the result of the studies indicates that age does not have a significant prognostic effect in determination of ges-tation length.
Sex or twins effected gestation length. The mares with colts were carrying in the aver-age 3,41 days longer that those carrying fillies (P<0.05). Average gestation of twins was 326,86 days, 5,51 days shorter than in a single fetus (P<0.01). The reason of this phe-nomenon is unknown with some reports not confirming finding a difference 3.15. The year of breeding considerably (P<0.01) effected gestation length. (Tabl. 4) e. g. the average gestation length of 1991 was 9,1 days longer than the following year 1992. Several authors confirm the effect of the year on gestation length 6,11,15 althougt they all confirm that it is impossible to make an objective conclusion about the causes of the year effect. We think, in correspondence with other authors 5,6,13 that differences in feeding, pasture, as well as the weather effect can be one of the reasons for the differ-ences between some years in our study.
The study confirmed a strong influence of the breeding and foaling season on gestation length (Tabl. 5, 6). Mares bred in months January - May had gestation period as many as 10 days longer than those bred in months July - September. It is obvious from empirical knowledge that foalings in autumn occur after shorter gestation than those in spring 5. Similarly parturitions occurring in our study in late winter and early spring months (January - April) were connected to longer gestation, up to 13 days compared to parturitions in summer and early autumn (July - October) P<0.01.
Du�ek and Richter(6) state that the interpreting curve of gestation length has sinus shape in the course of the year. In accordance with the author we can state that in the study after gradual extending in spring months and reaching maximum in the months March and April decrease occurs. After the shortest gestation period - August, which practicaly overlaps with the end of grazing period (early September) there is again gradual extending of gestation until January.
The same conclusions were derived by Richter and Du�ek(13) with the differences in gestation lengths explained by a change in nutrition. A similar phenomenon in Australia was described by Rophila et al.14, who stated that shortening of gestation period can be caused by the length of days, as nutrition did not prove as the main factor in this study.
In another study of Arabian mares in Egypt gestation was longer at winter foaling. It seems that mares are able to some extent to adapt gestation length so that parturition tends to occur in spring. Such adaptation, though unrecorded, can be important for sur-vival of the species. The photoperiod effect on gestation length was demostrated by Texas workers12. The treatment began on 1 st. of December, the mares with fixed 16 hours photoperiod had reduced gestation length, 10 days, without foals being effected in a significant way. Photoperiod effect - the length of the day together with nutrition and climate seems the most acceptable explanation of the strong relation berween gestation length and the month of breeding or foaling.


Table 1.
Age of dam effects on length of gestation periods as differences from eight - year - old dams



No signifikant differences P> 0.05


Table 2.
Relation of gestation lenght to its number in life of the mare



Relation to the 1 - st gestation, no significant P >0.05


Table 3.
Relation of gestation length to the sex of the foal



Significant differences P< 0,05, twins P< 0.01


Table 4.
Year of breeding effects on length of gestation as differences from year 1965



a P< 0,01 b< P 0,05
Table 5.
Month of breeding effect on length of gestation as differences from April breedings



 

Table 6.
Month of foaling effect on length of gestation as differences from March foalings



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