By Dr Jon Kelly BVSc (Hons)
The use of sex- sorted semen by dairy farmers as a strategy to increase heifer replacement rates is being used more commonly. Recent technological advances in the process of producing sex-sorted semen by Sexing Technology (ST) has reportedly increased the underlying fertility of the semen.
This analysis details conception rates that have been achieved in heifer and cow herds in the Warrnambool district, with sex-sorted semen, using frozen sexed “XY”, fresh “Sexed UltraTM” and frozen “Sexed UltraTM”.
The factors that increase the chance of a successful conception, in heifers and cows, will also be discussed.
Types of sexed semen
“XY” sexed semen
The traditional method of producing sexed semen by ST, prior to June 2013, has been called “XY” sex-sorted semen. “XY” sexed semen is the product that we have the most knowledge of as it has been available for about 10 years in Australia. Frozen sex-sorted “XY” was thought to have the equivalent fertility to frozen conventional (non sex-sorted) of around 75%. In other words, if the normal conception rate for a bull was 50% with non sex-sorted semen, then you could expect to achieve about 37% with the same bull frozen.
The equivalent fertility rate of 75% is an average. Factors that are not yet well understood effect the fertility of the sex-sorted semen product of a particular sire, meaning the equivalent fertility may range from 50- 90%. This inconsistent fertility of the sex-sorted “XY” semen meant that widely variable conception rates were achieved on farm, even when all the other factors that make a successful pregnancy were accounted for.
“Sexed UltraTM” sex-sorted semen
Since June 2013 a review of the entire process of producing sex-sorted semen has been undertaken and implemented by ST. The changes implemented have been aimed at increasing the underlying equivalent fertility of the sex-sorted semen product. The new sex-sorted semen product, either frozen or fresh, is called “Sexed UltraTM”.
Recent data released by ST suggests that the equivalent fertility of frozen “Sexed UltraTM” is about 95% to that of frozen conventional semen.
Fresh “Sexed UltraTM” semen has also been available in Australia over the last 2 years. The benefits of fresh semen should be that more “live” semen is deposited into the uterus, as the semen has not needed to go through the traditional freeze/thaw process that damages sperm cells.
Factors that affect fertility in heifers
The most fertile animal on a dairy farm is a well grown heifer, free of any reproductive disease. This group are the most obvious choice for the use of sex-sorted semen. The most common disease in heifers that could cause decreased reproductive performance is Bovine Viral Diarrhoea Virus (BVDV) or Pestivirus. The infection status of a group is easily determined, and appropriate preventative strategies should be completed prior to joining.
A recent study conducted by Warrnambool Veterinary Clinic (Noonan et al. 2015), showed that the weight of the heifer, not age, or any other factor related to the Fixed time AI program used, showed the strongest correlation with expression of oestrus at the time of AI. Not surprisingly, expression of oestrus at the time of FTAI (using this particular 10 day program) by a heifer, gave a 3.4 times better chance of a successful conception with frozen sex-sorted “XY” semen.
This study reinforced that puberty, and sexual maturity, is related to weight, not age, and that it is critically important for heifers to have achieved target weights at joining.
Synchrony or natural joining program?
When giving advice on the “best” joining program for heifer mobs using sex-sorted semen, the success of historical methods for joining heifers is usually the best guide as to what method should be chosen.
This is based on the observation that any small increases in conception rate that may be gained through one method compared to another, is usually outweighed by the unfamiliarity of a “new” program for that farm.
Natural heat and Prostaglandin programs rely on consistent and accurate heat detection, with facilities and staff needing to be able to cope with the all the factors that may interfere with this.
Fixed Time AI (FTAI) programs, that have pre- determined events and AI, are used in heifer mobs when heat detection is not able to be consistent or possible.
There are many FTAI programs currently available, but the program WVC is most familiar with, and we think gives the most consistent conception result with sexed semen, is a 10 day CIDR insert program (WVC FTAI).
Sex-sorted semen Conception results
The following table describes conception rates achieved in Warrnambool district heifers using sexed semen.
Although the number of heifers, and sires used, analysed for frozen “Sexed UltraTM” is only an early indication, it would seem that there is a general trend of increased conception rates compared to frozen “XY” sex-sorted semen. The conception rate achieved for frozen “Sexed UltraTM” seems to be equal to that of Fresh “Sexed UltraTM”. If this proves to be true, the ability to use frozen sex-sorted semen without compromising conception rates, compared to Fresh sexed semen, is a significant benefit.
Factors that affect fertility in cows
It needs to be recognised that the inherent fertility of the cow is, on average, less than the heifer.
Hence, when sex-sorted semen is considered for use in the cow herd, consideration must be given as to the cumulative effects of using a semen product with lower equivalent fertility to conventional semen. Less cows may get in calf as quickly this season, which will affect the calving pattern and subsequent reproductive performance of the cow next year.
Cow factors that decrease fertility
- Body Condition Score (BCS) – Cows that lose more than one BCS from calving to joining, or are less than BCS 4.5 at Mating Start Date (MSD).
- Age – generally cows over 6 year old
- Days In Milk – Cows calved less than 40 days prior to MSD
- Reproductive disease - BVDV
- Health events
- Events related to the reproductive tract (e.g retained membranes, metritis, endometritis) (dirty cows))
- Difficult calving
- Metabolic disease at calving (milk fever or pregnancy toxaemia)
- Illness at or prior to joining.
Synchrony or natural joining program?
Many of the cow factors that are implicated in reduced fertility are also negatively associated with cyclicity, or No Visible oestrus (NVO). FTAI synchrony programs, that are not able to differentiate cycling from NVO, generally have lower conception rates with conventional semen, so sex-sorted semen is not advised for use.
Hence simply only selecting cows for sex-sorted semen that show a natural heat (either natural or prostaglandin induced) is recommended when using sex-sorted semen. Using a prostaglandin based synchrony program also has the benefit of identifying and joining cycling cows sooner than a natural program, but also allows early identification, and treatment, of NVO cows.
Sexed semen Conception results
The following table describes conception rates achieved in Warrnambool district cows using sexed semen.
Although the number of cows, and sires used, analysed for frozen “Sexed UltraTM” is only an early indication, it would seem that there is a general trend of increased conception rates compared to frozen “XY” sexed semen. The conception rate achieved for frozen “Sexed UltraTM” seems to be equal to that of Fresh “Sexed UltraTM”.
It needs to be noted however, that most of the cow conception rates analysed are on “high fertility selected cows”, not a herd average conception rate. By selecting these cows we are artificially inflating the “average” conception rate and thus should not be viewed as expected results if sex-sorted semen was to be used on the whole herd.
Noonan, E., Kelly, J. & Beggs, D., 2015. Observations on the fertility of nulliparous dairy heifers following a 10 day fixed-time artificial insemination program with sex-sorted and conventional semen. AVJ, in press.