| 1200 sows in all were used in this experiment,which contain 925 Landrace×Yorkshire or Yorkshire×Landrace,54 Landrace,223 Yorkshire.The aims of this study were to investigate back fat levels in sows during whole reproductive periods and to assess whether they were associated with average feed intake during latation and reproductive performance.And the correlation between back fat level and average feed intake during lactation was also assessed.Finaly,the optimal reproductive performance can be reached by controling nutritional stratage during whole reproductive periods.The results indicated that:(1) For Landrace×Yorkshire or Yorkshire×Landrace sows,litter size during lactation and back fat thickness at mating has a significant positive association with VFI during lactation(P＜0.01),and a significant negative association was found between lactating month and VFI(P＜0.01),between back fat thickness at 30 days during gestation and VFI(P＜0.05).The effect of lactating month(x3),litter size during lactation(x4) or back fat lost during lactation(x15)was illustrated by regression analyses of VFI during lactation(y1):y1=4.3635-0.1565x3+0.1219x4-0.0694x15(n=217,R2=0.2757,Root MSE=0.7139,Radjust2=0.2655,C.V.=16.63,P＜0.0001).(2) The effect of parity,lactating month,litter size during lactation,back fat thickness at mating on voluntary feed intake during lactation were significant(P＜0.01),and the back fat lost during lactation(P=0.0563) and back fat level at 30 days during gestation(P=0.0618) tend to increace voluntary feed intake.(3)Reproductive performance such as interval between weaning to mating and total number of piglets born per litter,number of piglets born alive and litter weight was influenced significantly by back fat level.Total number of piglets born per litter was tended to effeced by back fat thickness at mating(P=0.0681);Interval between weaning to mating was effeced by back fat level at 30days during gestation(P=0.0459),and total number of piglets born per litter tended to effected by him too(P=0.0814); Number of piglets born alive was effected by back fat level at farrowing(P=0.0498);Change of back fat during gestation have a significant influence on total number of piglets born per litter(P=0.0314),number of piglets born alive (P=0.0398) and litter weight(P=0.0466).Optimal litter performance can be obtained by assure that the back fat during whole gestation phases is plain.(4)Factors who have an effect on annual fertility were illustrated by regression analyses of annual fertility(y3):y3=32.8680-9.4078x4+0.2261x13+0.3681x42-0.0106x92+ 2.1940y5+2.6893y12-0.1242y2(n=198,R2=0.8856,Root MSE=2.9431,Radjust2 =0.8814,C.V.=14.42,P＜0.0001)(x4,litter size during lactation(head);x13,back fat thickness at weaning,(mm);x9,back fat thickness at 90 days during gestation,(mm); y5,piglets born alive,(head);y12,litter size at weaning,(head);y2,interval between weaning to mating,(day))(5) Average daily gain weight during lactation(P=0.0259)and litter size at weaning(P=0.0306) were effected significantly by average feed intake during lactation. Average feed intake during lactation tend to effect interval between weaning to mating and annual fertility of sows(P=0.0715,0.0675).In order to get an optimal reproductvie performance,it is best to reach 46.10～65.85MJ/head of DE every day.
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