Understanding gestation length and how it is used in the dairy industry
Knowledge of expected calving dates is important for a number of herd management practices such as spring feed budgeting, mob allocation, and predicting when things will be busy on farm.
In the industry gestation length has traditionally been considered to be approximately 282 days; the length of time between conception and the birth of the calf. MINDA® uses gestation length information to make expected calving reports more accurate than they were historically to help farmers make more informed management decisions.
Part of a cow’s expected calving date is based on her own gestation length (as well as the gestation length of the calf’s sire) and this varies between individual animals because of genetics as well as other factors like calf gender and dam age:
Gestation length by calf gender
Recent research (Donkersloot, 2014) on more than 58,000 New Zealand animals found the average gestation length to be:
Gestation length by dam age
In the same study, the average gestation length was shorter in first-calving cows, being 279.5 days for first calvers, and 280.8 days for second calvers. This has also been reported in King et al. (1985), and Dhakal et al. (2013).
Normal variation around gestation length
Research at LIC has examined the variation in gestation length. 95% of animals are expected to give birth within 9 days of their expected calving date. For example, if 100 heifers were expected to calve on 20th July, 95 of them should calve between July 11th and 29th. The other five heifers are expected to calve outside of this window.
Common questions around gestation length
Is the national herd calving earlier each year?
Yes. Compared to the average gestation length of New Zealand’s 2005-born cows (genetic base cow, which was 281 days), the average gestation length of all herd tested cows was 1.3 days shorter in July 2016.
Are my cows calving earlier each year?
Whether your cows are calving earlier each year is not entirely dependent on gestation length, but also on when your planned start of calving (PSC) is. PSC dates have been getting earlier in the majority of regions over the last 15 years. See Figure 1.
Figure 1. Trend in planned start of calving dates for cows (excluding first calvers) by region (Livestock Improvement Corporation Limited and DairyNZ Limited, 2015)
Sire breeding values for gestation length are becoming slightly more negative over time. Consider the average gestation length BV of the bulls you are using in your herd.
If you have cows with negative gestation length breeding values and these are mated successfully to bulls with negative gestation length breeding values then the replacement heifers will have negative gestation length breeding values as well. Slowly the gestation length breeding values of your herd will be becoming more negative, leading to shorter gestation lengths.
This shorter gestation length can be advantageous as it gives cows more time to recover from calving before mating starts, but it does mean that you will need to ensure you have adequate feed on the milking platform for the early calvers.
What does MINDA use to calculate expected calving dates?
To calculate expected calving dates, MINDA reports use both sire and dam gestation length breeding values.
What are gestation length BVs?
The BV for gestation length estimates a cow or bull’s genetic merit for gestation length, and estimates this from differences between their progenies’ expected calving date and their actual calving date.
How does MINDA use gestation length BVs to calculate expected calving dates?
Progeny inherit half their genetics from their dam, and half from their sire; this goes for gestation length too.
If a sire had a gestation length BV of -4 days and the dam had a gestation length BV of -2 days then you would expect the sire to shorten the gestation length by 2 days, and the dam by 1 day, meaning the cow would likely calve 3 days earlier than the standard 282 days.
For a cow carrying a single calf, the expected calving date would be calculated as:
What does Animal Evaluation use gestation length for?
Animal Evaluation uses gestation lengths in determining which bull sired which calf.
The rules around whether a sire is assigned to a calf change based on calf gender and whether the calf is a single or one of multiple birth. In Animal Evaluation the assumed gestation lengths are 282 days for heifer calves, 283 days for bull calves, and 276 days for multiple births.
Where can I find out about the gestation length BV of sires I use?
The easiest way to check gestation length BVs of the sires you use is to view the bull catalogue from your AB provider. Each bull will have their gestation length BV documented, as shown below.
Do different breeds have different gestation lengths?
Yes. In New Zealand the average gestation length BV of herd tested cows of various breeds in July 2016 is:
Are there expected calving distributions (spreads) for cows?
Yes. Research completed in 2001 and 2014 on New Zealand animals determined the distribution of gestation length.
Winkelman and Spelman (2001) found average gestation lengths of:
Donkersloot (2014) found average gestation lengths of:
95% of calvings are expected to fall within two standard deviations of the average.
Do short gestation length (SGL) sires have tighter calving spreads or are they still subject to the same calving distribution curves?
This has not been investigated yet. LIC scientists hypothesise that the calving distribution is likely to be slightly smaller, but the main difference will be a downward shift in distribution– so an average of say 276 days ± 9 days, rather than 282 days ± 9 days.
Now that we have gestation length BVs, does the ± 11 days still apply or is it shorter now?
For assigning sires, AEU uses the ± 11 days around the estimated mating date (which is adjusted based on sire gestation length BV). This process won’t change unless the sire determination methods are changed within AEU.
Work is currently being undertaken to investigate and modify the sire determination process to also account for the dam’s gestation length. Recognising the reduction in gestation length of the national herd and that some individual cows have gestation length breeding values as low as -8 days.
For AB sires it is likely that the ± 11 day rule is a good approximation. However, for SGL sires the variation is likely to be smaller due to them being selected specifically for gestation length, but we don’t have any numbers to put around this.
Prepared by Charlotte Gray, Lorna McNaughton, Esther Donkersloot & Anne Winkelman
Livestock Improvement Corporation Limited & Dairy NZ Limited. (2015). New Zealand Dairy Statistics 2015-15. New Zealand: Hamilton
Donkersloot, E. (2014). Association mapping of gestation length in New Zealand dairy cattle (Master’s thesis, Wageningen University, Wageningen, Netherlands)
Winkelman, A.M., & Spelman, R.J. (2001). Selection for reduced gestation length in New Zealand dairy cattle. Association for the Advancement of Animal Breeding and Genetics, 14:63-66.
King, K.K., Seidel, G.E., & Elsden, R.P. (1985). Bovine embryo transfer pregnancies, II. Lengths of gestation. Journal of Animal Science, 61: 758-762.
Dhakal, K., Maltecca, C., Cassady, J.P., Baloche, G., Williams, C.M., & Washburn, S.P. (2013). Calf birth weight, gestation length, calving ease, and neonatal calf mortality in Holstein, Jersey, and crossbred cows in a pasture system. Journal of Dairy Science, 96: 690-698.