We looked at the data to see what changes – if any, would
come from herds changing from an average of 4 lactations per cow to 5.
Changing the average number of lactations drops the
replacement rate from 25% to 20%. Different energy and production levels for
cows at each age group was accounted for. A farm scenario with 14 tonnes of dry
matter eaten per hectare per year, including replacement stock was modelled.
Production, enteric methane and urinary nitrogen were
calculated at a farm level (expressed per hectare) and at a product level
(expressed per KgMS). New Zealand Greenhouse Gas Inventory energy and emissions
modelling equations were used.
With the lower replacement rate for the 5 lactations
scenario, there is a greater proportion of feed available to milking cows
compared to the 4 lactations scenario.
This results in 2.62 milking cows per hectare for the 4 lactations
scenario and 2.67 milking cows per hectare for the 5 lactations scenario.
Farm production was
4.4% higher for the 5 lactations scenario compared to the 4 lactations
scenario. The greater number of lactations results in a higher proportion of
mature cows producing to their full potential rather than young cows that are
still using energy for growth.
Enteric Methane Emissions:
There was no difference in enteric methane emissions at a
farm level as the same amount of dry matter is assumed to be eaten in both
scenarios. Enteric methane production is directly related to total dry matter
intake regardless of the age of the dairy cows, and this is constant across
There is 4.2% less enteric methane emissions per kilogram of milk solid produced
for the 5 lactations scenario as there is higher farm production with no increase total
methane emissions from the farm.
Urinary Nitrogen Excreted:
Urinary nitrogen excreted has a similar trend as enteric
methane emissions but with some differences. The amount of nitrogen retained by
the cows for growth and the amount of nitrogen secreted as milk protein varies
with age. On a per hectare basis there is 0.6% less urinary nitrogen excreted
when for the 5 lactations scenario. The nitrogen intake going to milk
production is proportionally greater than that retained for growth for the
For the 5 lactations scenario there is 5.2% less urinary
nitrogen excreted per kilogram of milk solid as there are higher production
levels because of the higher mature cow stocking rate and higher production
level per cow.
- To ensure the rate of genetic gain is not slowed
down when the herd has a greater number of lactations, consideration should be
given to targeting more replacements from the younger cows of the herd. Less
replacements will be required so there are options available to ensure this can
- The higher replacement rate scenario will likely
have more cows being sold for meat processing each year so more emissions could
be allocated towards meat production. But, the lower replacement rate scenario
has the option for more dairy beef calves to be generated which could be more
efficient at growing beef with lower emissions profile than cull cows.
- The urinary nitrogen is expressed as an annual
total, the scenarios didn’t show much difference in urinary nitrogen total deposited
in high risk months for nitrogen leaching from February to July. However, the
size and quantity of urine patches and therefore the risk of nitrogen leaching
has not been calculated.