EMBRYO MORTALITY – THE HIDDEN BOGEYMAN!
The month of May will be remembered as one of the harshest on record. Temperatures reduced grass growth rates and cows had to be rehoused by night in many wetland areas.
Farmers had to resort to increasing concentrate supplementation to maintain milk yields and avoid body condition score loss. Unfortunately, in many cases the required interventions did not occur.
With increased stress on cows at a critical period of spring calving breeding programmes, reports of poorer heat detection or inordinately high non-return rates to first service were the first signs of a lurking bogeyman!
Embryo development encompasses, in the first instance fertilisation 24 to 36 hours after the onset of heat. The sperm that fertilises the egg has to be in the oviduct at the point of fertilisation for a minimum period of six hours. The latter is known as capacitation. A sample analogy of this process is the equivalent of a motorcyclist covering a distance of 100 miles in 20 minutes and a period of 6 hours to take off his helmet at the finish line.
If both oocyte and sperm are competent, fertilisation takes place in approximately 90pc of cows. The early development of the embryo incorporates cell divisions to farm 2, 4,8,16 and 32 cells. The early divisions are dependent on nutrient reserves of the oocyte. Some embryos fail to get through these early stages and referred to as 4 to 12 cell stage blocks to development. It is noteworthy that the individual cells of these young embryos are totipotent. This means that each of the individual cells of an 8 cell embryo can be transferred to other empty zygotes to form identical offspring.
Early embryo mortality with death prior in the 2 to 32 cell stage is primarily associated with events prior to breeding. Therefore, stressors associated with nutrient quality and supply will impact negatively on egg quality.
Optimisation of reproductive performance has to be your primary concern during a breeding programme. The temptation to reduce costs in a low milk price scenario can backfire with significantly greater costs associated with higher empty rates with a restricted 9 to 12 week breeding period.
Therefore, avoid any risk of nutritional imbalances as a flush of grass with increased temperatures are associated with highly degradable grass proteins. Supplement with a concentrate balancer which will mop up impact of excess nitrogen.
The next hurdle to embryo development occurs as the embryo hatches between day 7 and 8 after breeding through day 13 after which maternal recognition occurs. This challenge is akin to running the marathon in less than 2 hours 20 minutes!
The key to success is the ability of the trophoblast which surrounds the embryo to grow and elongate with sufficient prosperity to signal the presence of the embryo to the mother by day 13. The uterine environment dictates the growth rate of the trophoblast. Once again, avoid any stressors (mastitis, lameness, golf ball grazing), which will compromise early embryo survival. Having succeeded in telling the mother “I am here”, the embryo must continue elongation in a ‘tapeworm’ effect to complete implantation by day 34 after breeding. At this stage the embryo is referred to as a foetus.
Embryo death prior to day 13 after breeding does not affect the normal return to heat. However, later embryo death is the real ‘bogeyman’ as embryo death is associated with the maintenance of a pregnancy ‘state’. This can last for periods up to nine weeks.
Recent research has identified critical risk periods for embryo death prior to implantation. Twinning is also associated with greater embryo death.
It is essential that you scan your cows now prior to the end of the breeding season to identify and prevent the impact of the hidden ‘bogeyman’.
Dr. Dan Ryan is a bovine reproductive physiologist and can be contacted at www.cowsdna.com
Article written by Dr. Dan Ryan in the Farming Independent 16th June 2015