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.

Stocking rates:

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.

Production:

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 both scenarios.

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 replacement stock.

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.

Additional considerations

  1. 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 happen.
  2. 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.
  3. 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.