Pressure from government, industry, processing companies and the general public is mounting for dairy farmers to reduce their greenhouse gas emissions. But how can dairy farmers do this, and remain profitable in an economic climate where margins are continually squeezed? Tools that are commonly talked about such as dietary methane inhibitors, vaccines and genetic selection for low methane emitting animals are still a long way off and will come at a cost.

The metric for dairy farmers that relates directly to atmospheric warming is the amount of methane emitted per hectare. This is the metric that, as an industry, dairy is being asked to reduce 10% of 2017 levels by 2030 and 24-47% by 2050.

Currently, the only way to significantly reduce methane emissions/ha is to reduce dry matter (DM) intake/ha. For every kg DM a cow ingests, approximately 21 g of methane is emitted as a result of rumen microbial digestion. Decreasing cow numbers but not decreasing total DM intake will not reduce methane production. Decreasing DM intake without making any other change will result in a decrease in milk solids production and, most likely, a decrease in profitability.

So, is there a way to significantly decrease methane production by reducing total DM intake but without decreasing milk production and profitability?

The answer is yes, for dairy herds where their average lactating cow liveweight has room to decrease through breeding (e.g. liveweight above 450 kg). By decreasing the liveweight/cow through breeding, cow numbers can stay the same, production will stay the same, but feed required will be lower as a result of less liveweight/ha. Essentially less feed is required because the amount required for maintenance is less. Production can be maintained by feeding the cows to the previous level of milk solids production.

Here is an example:

 Herd A: 525 kg cows producing 400 kg MS/year

Replace with Herd B: 425 kg cows and feed to produce 400 kg MS/year. Same number of cows as Herd A but 100 kg lighter.

Based on DairyNZ Facts and Figures:

Herd A cows require 5.2 ton DM/year at 11.0 MJME/kg DM to produce 400 kg MS/cow/year.

Herd B cows require 4.7 ton DM/year of the same ME feed.

Herd B therefore requires 0.5 ton DM/cow/year (5.2 - 4.7) less feed to produce the same milk solids.

Industry uses a standard figure of 21 g of methane emitted per kg DM eaten (21 kg/ton DM)

Cows in Herd A will emit 5.2 ton x 21 kg = 109 kg methane/cow/year (2725 kg CO2equiv)

Cows in Herd B will emit 4.7 ton x 21 kg = 98.7 kg methane/cow/year (2467 kg CO2equiv)

By breeding lower liveweight cows, this farm would decrease its methane emissions by 9.5%, will produce the same amount of milk solids and likely be more profitable if less supplement is purchased or less nitrogen applied.

The table below demonstrates the expected reduction in methane/cow/year and per hectare/year (if cow numbers stay the same) by decreasing average cow liveweight by 50 kg, 100 kg or 150 kg.  Even a liveweight reduction of 50 kg (e.g. from 470-420 kg liveweight) results in a significant fall in methane emitted.

* Calculated from DairyNZ Facts and Figures 4. Cow Feed Requirements page 46.

Along with the decrease in methane emitted, there will also be a drop in the other main agriculture-related greenhouse gas, nitrous oxide (N2O). Two of the big drivers of N2O emissions are quantity of protein and nonprotein nitrogen ingested and quantity of nitrogen fertiliser applied to pasture. Clearly, reducing total feed intake will lower the protein and nonprotein nitrogen ingested. Also, if less feed is required, it is likely that less nitrogen will be applied to pasture.

 In summary:

Breeding lighter cows is a simple, no cost way to reduce greenhouse gas emissions without impacting on production and will probably increase profitability. Decreasing average herd liveweight by 100 kg reduces emissions by approximately 10%, which is the 2030 industry target.