The aim at this study was to determine the relationships among the lamb birth weight, the average cotyledon surface area (ACSA) and cotyledon size. Data were collected from 101 ewes. The general linear model and Pearson correlation coefficient were used for statistical comparison and determination of relationships between variables. Average birth weight (BW), placental weight (PW), cotyledon number (CN), placental efficiency (PE), cotyledon density (CD), cotyledon efficiency (CE) and ACSA were 4.175 ± .09 , 448.8 ± 13.4 , 53.34 ± 1.9 , 9.65 ± .3 , 0.125 ± .00 , 10.66 ± .34 and 7.81 ± .19 cm 2 , respectively. There was no difference between BW and PE for parity; however, PW, CN, CD, CE and ACSA were affected ( p < 0.05 ) by parity. ACSA was found to be the lowest ( 7.33 ± .99 ) with a parity of 2 and the highest ( 8.61 ± 1.5 ) with a parity of 4. Birth type affected significantly BW, CN, CD ( p < 0.05 ), PW ( p < 0.001 ) and ACSA ( p < 0.01 ). As the parity progressed, cotyledon depth (CDe) ( 0.74 ± .30 ) and cotyledon width (CWi) ( 2.64 ± .46 ) increased. ACSA, which is a new parameter for uterine capacity, had positive correlations with BW (0.498; p < 0.01 ), PW (0.415; p < 0.01 ), large cotyledon number (CNl) (0.685; p < 0.01 ), cotyledon length (CL) (0.932; p < 0.01 ), CWi (0.920; p < 0.01 ) and cotyledon depth (0.388; p < 0.01 ). The most important finding of this study was the positive correlation between the birth weight and the average cotyledon surface area. This study indicates that average cotyledon surface area and cotyledon size traits (CL and CWi) may be more effective parameters to produce heavier lambs. In conclusion, it is thought that lamb deaths will decrease as a result of triggering placental development with proper feeding during pregnancy. For this purpose, it is recommended to conduct new studies examining the relationship between pregnancy feeding and ACSA. Highlights. Cotyledon number and cotyledon density decrease with parity, while CE increases.The birth type has a significant effect on BW, PW, CN and CD.There are positive correlations between the placental weight and large cotyledon number, cotyledon length and cotyledon width.ACSA can be used as an important parameter to increase the weight of lambs.Especially in multiple birthing, ACSA has been found to be a more determinant index to express uterine capacity instead of PE or CE.

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