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  • Writer's pictureRhiannon Handcock

Do herds milked once daily have better 6-week in-calf rates than twice daily herds?

Updated: May 25, 2020

The main conclusions from that research summary was that although there was a reduction in milk production for OAD herds compared with TAD herds in industry, the difference was less than that for experimental research studies.

This goes to show, on a whole farm level, changes to milking frequency from OAD to TAD can be utilised somewhat successfully.

A reason for switching from TAD to OAD full season (other than spending half as much time in the milking shed!) may be an improvement in reproductive performance.

This week's study is a comparison of reproductive performance for herds milked OAD or TAD for the full season.


Key Points:

  1. Herds that were milked once-a-day all season had a 10.4% higher 6-week in-calf rate than twice-a-day herds.

  2. The average not-in-calf rate was 12.8% for OAD herds and was higher at 17.6% for TAD herds

  3. Perhaps for some farms, making the switch to OAD from TAD may be a help improve reproductive performance. But, it is definitely not a silver bullet!


Did once-a-day herds have better 6-week in-calf rates than twice-a-day herds?

The once-a-day herds had a 10.4% higher 6-week in-calf rate than the twice-a-day herds.

As can be seen in the graph below, herds that milked OAD all season had an average 6-week pregnancy rate of 74.8% (blue).

This average was made up of 3 seasons of data, from 2014/15 to 2016/17 seasons. In those years the national average 6-week in-calf rate was between 65.6% and 66.8% (page 40 of the Dairy Statistics).

Figure 1. Average six-week in-calf rate of herds milked once-a-day (OAD) or twice-a-day (TAD) all season

The TAD herds had a 6-week in-calf rate of 64.4% (purple) that was slightly lower than these national averages.

Both the averages for OAD and TAD herds were lower than the target of 78%.

The authors make a point that these results do not mean that TAD herds cannot achieve great reproductive performance. But for some herds, a switch to once daily may be beneficial to herd reproductive performance.

Interestingly, the OAD herds had a shorter on average mating length of 71.8 days (approx. 10 weeks) than the TAD average of 76.1 days (approx. 11 weeks).

Did once-a-day herds have lower not-in-calf rates than twice-a-day herds?

Despite the shorter mating length, the average not-in-calf rate of OAD herds was 4.8% lower at 12.8% than for the TAD herds at 17.6% (see graph below).

The not-in-calf rate is defined in the InCalf Book as "the percentage of the herd that failed to become pregnant during both the artificial breeding (AB) and bull mating periods". At any time point, it is calculated as 100% minus the in-calf rate.

Figure 2. Average not in-calf rate (ICR) of herds milked once-a-day (OAD) or twice-a-day (TAD) all season

The not-in-calf rate (previously referred to as the empty rate) is a difficult measure to use to compare performance among herds. This is because as mating length increases, the not-in-calf rate (generally) decreases. This is why 6-week in-calf rate is the most common measure of performance used in NZ.

What drives the 6-week in-calf rate?

Two major drivers of the 6-week in-calf rate are 3-week submission rate (proportion of cows submitted in first 3-weeks of mating) and conception rate (proportion of inseminations that resulted in a pregnancy).

The target 3-week submission rate is 90% and the target conception rate is 60%. If we multiply the 2 together we get the target 3-week in-calf rate (90% x 60% = 54% 3-week in-calf rate) --> for more on targets and advice check out the in-calf resources from DairyNZ.

For the herds milked OAD all season, their 3-week submission rate was 84.6%. This was 7.7% higher than the 3-week submission rate for the TAD milked herds (Figure 3).

Figure 3. Average 3-week submission rate (SR) of herds milked once-a-day (OAD) or twice-a-day (TAD) all season

Additionally, the OAD milked herds had a 7.9% higher average conception rate than the TAD herds (Figure 4).

The higher submission rate and conception rate for the OAD herds was a major contributor to the higher 6-week in-calf rate compared with the TAD herds.

Figure 4. Average conception rate (CR) of herds milked once-a-day (OAD) or twice-a-day (TAD) all season

Overall, this resulted in 4.3% more of the herd calved by the sixth week of calving in the OAD herds than the TAD herds.

For a 400 cow herd, this would be equivalent to 17 more cows calved in the first 6 weeks for the OAD herds.

Figure 5. Average six-week calving rate of herds milked once-a-day (OAD) or twice-a-day (TAD) all season

These results indicate that herds milking OAD for the whole season have, on average, better reproductive performance than those milked TAD the whole season.

Although, as mentioned earlier, just because a herd is milking twice daily, it does not mean that they cannot achieve great reproductive performance. Additionally, just because a herd switches from twice daily to once daily it does not mean that they will automatically improve their reproductive performance.

Herd reproductive performance is a tricky thing to get really great results with, perhaps for some farms, making the switch to once-a-day may help.

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What data were used?

Herds were classified as once-a-day or twice-a-day if they herd tested 3 or more times a season and more than 90% of animals had the same milking regime (OAD or TAD) at each test.

Once the OAD herds were identified, the closest geographical TAD herd was identified and included in the dataset for the 2014 to 2016 seasons.

Only herds with a Detailed InCalf Fertility Focus report were analysed (OAD n=75, TAD n=76).

There were slight differences in the breeds and herd size of the OAD and TAD herds.

On average there were 400 cows in the OAD herds and 445 cows in the TAD herds. The authors concluded that it is unlikely that the size of the herd had much of an impact on reproductive performance.

Further, the OAD herds had more Jersey and crossbred cows than the TAD herds did. In general, crossbred cows have slightly better 6-week in-calf rate than pure Friesians or Jerseys. It is unlikely that the differences in breed makeup between the TAD and OAD herds studied was enough to explain all the differences observed in this study.

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