What's up with my empty rate?

Every year once the pregnancy test results are in we often hear “empty rates are up this year” and “empty rates are way higher than they used to be”. The question is – are these statements true?

Are empty rates are up this year?

Using the spring 2017 mating results1, we looked into the national average not-in-calf rate2 (NICR).  The national average NICR for this year was 17%.  This is the same as it was for the 2015 and 2016 seasons.

So for the 2017 season, the NICR/empty rates have not increased, but remained the same.

Are empty rates are higher than they used to be?

We collated the last 6 season’s data3 to see if the national average NICR had gone up over time (Table 1).

Table 1: Average 6-week in-calf rate, not-in-calf rate and mating length for the 2012 to 2017 spring matings

6-week in-calf rate

Not-in-calf rate

Mating length

Spring 2012

6-week in-calf rate 67%

Not-in-calf rate 14%

Mating length 85 days

Spring 2013

6-week in-calf rate 67%

Not-in-calf rate 15%

Mating length 84 days

Spring 2014

6-week in-calf rate 67%

Not-in-calf rate 16%

Mating length 78 days

Spring 2015

6-week in-calf rate 67%

Not-in-calf rate 17%

Mating length 76 days

Spring 2016

6-week in-calf rate 66%

Not-in-calf rate 17%

Mating length 76 days

Spring 2017

6-week in-calf rate 66%

Not-in-calf rate 17%

Mating length 75 days

The national average NICR increased by 3% over the last 6 years.  Given the two main drivers of NICR are 6-week in-calf rate and mating length, we looked more closely into these two measures.

6-week in-calf rate

In general, the higher your 6-week in-calf rate, the lower your NICR rate will be.  This is illustrated by the 2017 spring mating results1 in Table 2 (quartiles based on 6-week in-calf rate).  In general, a 2% drop in 6-week in-calf rate results in a 1% increase in NICR (and vice versa).

Table 2: The relationship between 6-week in-calf rate and not-in-calf rate

Average 6-week in-calf rate

Average not-in-calf rate

Top Quartile

Average 6-week in-calf rate 76%

Average not-in-calf rate 13%

2nd Quartile

Average 6-week in-calf rate 70%

Average not-in-calf rate 15%

3rd Quartile

Average 6-week in-calf rate 65%

Average not-in-calf rate 18%

Bottom Quartile

Average 6-week in-calf rate 55%

Average not-in-calf rate 23%

Mating length

Mating length has a large influence over NICR - the longer you mate for, the more cows that will be in calf by the end of mating. 

For a herd with an average 6-week in-calf rate (66%), Table 3 shows the expected NICR for 6 different mating lengths4.  These figures show that when 6-week in-calf rate stays the same, NICR increases as mating length is shortened.

Table 3: The relationship between mating length and expected not-in-calf rate

Mating length

Expected not-in-calf rate

Not-in-calf rate increase

Mating length 13 weeks

Expected not-in-calf rate 15%

Mating length 12 weeks

Expected not-in-calf rate 16%

Not-in-calf rate increase +1%

Mating length 11 weeks

Expected not-in-calf rate 18%

Not-in-calf rate increase +2%

Mating length 10 weeks

Expected not-in-calf rate 20%

Not-in-calf rate increase +2%

Mating length 9 weeks

Expected not-in-calf rate 22%

Not-in-calf rate increase +2%

Mating length 8 weeks

Expected not-in-calf rate 25%

Not-in-calf rate increase +3%

Is a drop in mating length to blame for the rise in NICR?

Inductions were fully phased out in 2015.  To try and maintain a good calving pattern, many made the choice a season or two before 2015 to reduce how long they mated for.  This resulted in the national average mating length dropping from a little over 12 weeks (85 days) in 2012 to just under 11 weeks (75 days) in 2017 (Table 1).  Since 6-week in-calf rate stayed pretty much the same over that time, we’d have expected a 2% increase in NICR due to the shorter mating (based on Table 3).

It looks like two thirds of the reason for the 3% increase in NICR over the last 6 seasons is due to us shortening mating length by 10 days (without lifting our 6-week in-calf rate).

Using the rule of thumb that a 2% change in 6-week in-calf rate generally results in a 1% change in NICR, to compensate for the expected 2% increase in NICR, we’d have needed to lift our average 6-week in-calf rate by around 4%.

Thinking ahead

If you’re thinking about shortening your herds mating length in the future, make sure you have a plan to get more cows in calf in the first 6 weeks to help minimise the impact of reducing mating length.

To find out more about lifting your herd’s 6-week in-calf rate, check out the reproduction resources on our website.

1 Herds included in this analysis were 4,032 spring calving seasonal herds that had a 2017 Spring Detailed InCalf Fertility Focus Report.

2 Not-in-calf rate is the percent of the herd that have not been recorded as ‘pregnant’.  As well as cows recorded as empty, the not-in-calf rate includes cows without a pregnancy test result recorded and those still recorded as ‘doubtful’.  The ‘herd’ is the number of cows that calved that season and were still there at mating start.

3 Herds were included in the analysis of a seasons results (e.g. the spring 2012 season) if they were a spring calving seasonal herd that had a Detailed InCalf Fertility Focus Report.  The number of herds included ranged from 2543 herds for the 2012 season to 4032 herds for the 2017 season.

4 These figures are from the analysis of the 2016 spring mating results for 3,852 herds.  Herds were included in the analysis if they were spring calving seasonal herds that had a Detailed InCalf Fertility Focus Report.

Disclaimer: The reproduction measures analysed were calculated from data and information entered by herd owners and collected by LIC & DairyNZ.  Accuracy of the results reported here is subject to the accuracy of the data entered.