The animal article of the month for July is entitled ‘Relationships between ovulation rate and embryonic and placental characteristics in multiparous sows at 35 days of pregnancy‘

In the past few years, pig farms around the world have seen an increase in the total number of piglets born per litter. This has been achieved by genetic selection for litter size and by improved management of the sows. Associated with the increase in litter size, an increase is seen in the incidence of piglets born with a low birth weight and thereby a decrease in birth weight uniformity. This has become an economical and welfare concern for the pork production chain, because these piglets have a lower survivability during lactation and after weaning, requiring more labour, and also a compromised growth potential during the finishing phase.

The past genetic selection for increased litter size has resulted in a sharp increase in ovulation rate. The ovulation rate is the number of oocytes released at ovulation, which after fertilization represents the potential number of embryos and piglets in the uterus during gestation. Modern crossbred sows have a much higher ovulation rate (on average 25) than their litter size (on average 15-17). We hypothesised that sows with a higher ovulation rate would have a higher number of embryos during early pregnancy, with -as a consequence- smaller placentas and smaller embryos resulting from a lack of sufficient uterine space. Because foetuses depend on their placentas for their nutrient uptake, smaller placentas are expected to negatively influence foetal development and consequently piglet birth weight.

We observed that an increase in ovulation rate is related with an increase in the number of vital embryos in sows at one third (35 days) of pregnancy. However, every four ovulations resulted in only one vital embryo; so, the majority of the potential embryos were already lost at this stage of pregnancy. These embryonic losses appeared to happen both before and after uterine attachment, which suggests both a low embryonic quality and a lack of sufficient uterine space. Another indication of lack of sufficient uterine space is the fact that these embryos had a smaller implantation site and a smaller placenta, which are expected to negatively influence piglet birth weight.

An important conclusion of this paper is that ovulation rate has a direct effect on embryonic and placental development. The smaller placentas of the vital embryos at this stage might not provide sufficient nutrients for foetal development at a later stage and thereby decrease piglet birth weight. This is currently under investigation by our research group.

This article is freely available for one month
Relationships between ovulation rate and embryonic and placental characteristics in multiparous sows at 35 days of pregnancy
Authors: C. L. A. Da Silva, H. van den Brand, B. F. A. Laurenssen, M. L. W. J Broekhuijse, E. F. Knol, B. Kemp and N. M. Soede

The animal Article of the Month is selected by the Editor-in-Chief and is freely available for one month. View the recent selections