Selection for uterine capacity improves fetal survival

 Work at the University of Alberta has shown that in very prolific sows with a high ovulation rate, embryo survival and quality of piglets at birth may be compromised due to limitations in nutrient supply via the placenta. One solution to this problem is to select for uterine capacity rather than total numbers born. Recent work by the USDA’s Animal Research Center in Nebraska, suggests that this approach does indeed improve fetal survival and, as a result, litter size.

The researchers selected pigs over 11 generations on the basis of ovulation rate (OR) or uterine capacity (UC) and then looked at the number of fetuses at different stages of gestation for each of these lines and an unselected control line (CO).  Gilts had one horn of their uterus and the corresponding ovary removed at 160 days of age. After breeding they were slaughtered at 25, 45, 65, 85 or 105 days of gestation and ovulation rate and the number of live and dead fetuses recorded. Fetal and placental weights were also recorded. 

As might be expected, the ovulation rate of OR gilts was highest (18.0) compared to the UC and CO lines, with 15.0 and 14.0 respectively. However, as pregnancy progressed and embryo losses occurred, the UC gilts had better embryo survival (Table 1). In fact, at full term, the number of pigs produced by UC gilts was predicted to be 19.6% higher than for the OR gilts.

Table 1: Embryo numbers in gilts selected for ovulation rate or uterine capacity

                                                                                           Days of gestation

                                                                25                45               65               85               105

 Unselected control (CO)                            10.1              8.3              7.2              6.7               7.3

Ovulation Rate (OR)                                   13.4              8.3              7.9              6.5               6.7

Uterine Capacity (UC)                                10.2              9.0              8.5              7.5               8.0

In each line, the number of live fetuses at day 25 was approximately 72% of ovulation rate. Mortality to day 45 was greatest in OR, intermediate in CO, and least in UC. Reductions in live fetuses continued to occur from day 45 to 105, but line differences at day 45 were essentially maintained to day 105. Selection for uterine capacity improved fetal survival primarily during the time period between days 25 and 45. Relative growth rate of placental tissue during gestation suggested a relatively later growth pattern of placental tissue in the UC line.

WHJ comment:  The differences between the three lines are quite staggering. It can be seen from the table that, even though the OR line had a much higher number of live embryos at day 25, their numbers decreased much more rapidly than the two other lines. This suggests that selection on the basis of uterine capacity should result in more piglets born alive per litter.

 Reference:  B. A. Freking, K. A. Leymaster, J. L. Vallet and R. K. Christenson. Number of fetuses and conceptus growth throughout gestation in lines of pigs selected for ovulation rate or uterine capacity.  J. Anim Sci. 2007. 85:2093-2103. doi:10.2527/jas.2006-766