21 August 2012

3 minute read

Improving milk yield has a far greater effect on reducing greenhouse gas emissions (GHGE) from UK dairy herds than extending lactation length – findings from a recent study, carried out by scientists in Edinburgh and London.

“So the combination of higher milk yield and longer lactations may be suitable to reduce GHGE without the concomitant decline in fertility becoming a problem,” said Geoff Pollott from the Royal Veterinary College, who led the work.

Under its Climate Change Act of 2008, the UK Government is committed by 2025 to reducing national GHGE to 80 per cent of 1990 levels. In the agricultural sector, livestock systems are an important source of GHGE, particularly methane (CH4) and nitrous oxide (N2O).

The team’s work follows up the suggestion that the adoption of extended lactations in dairy systems could lead to reduced national GHGE. The assumption underlying this work is that the use of extended lactations could lead to more efficient production and less wastage by both reducing the number of stock required in the national dairy herd and improving the efficiency of the remaining cows.

Scientists in London and at the Scottish Agricultural College adopted a modelling approach, based on relationships between key variables derived from an analysis of national recorded data.

“First we analysed data from UK dairy herds to define the characteristics of lactations of different lengths. These results were then used to model new herd scenarios based on extended lactations, and the GHGE calculated for each scenario,” explained Dr Pollott.

Lactations of 305, 370 and 440 days were compared to the model parameters used and the model was set up to keep the current total annual herd milk output constant, rather than individual cow lactation output.

Although total lactation output may be higher with longer lactations it may not always hold that annual outputs of alternative lactation lengths are constant. A number of assumptions were made for the model and the sensitivity of the model to a range of parameter changes were tested; these included replacement rate (20, 22 and 25 per cent), persistency (-10, +10 and +20 per cent) and milk yield (bottom 10 per cent and top 10 per cent for each lactation length).

“We found that as lactation length increased the number of calves and followers declined. And herd emissions increased as lactation length increased,” said Dr Pollott. “Lower replacement rates resulted in lower emissions, but the relative decline in GHGE as replacement rates reduced was low.”

The sensitivity analysis of lactation persistency demonstrated that more persistent lactations resulted in lower GHGE for all lactation length scenarios. However, the best lactation length/persistency combination (440 days and 20 per cent more persistent lactations) only resulted in a 7 per cent reduction in emissions.

“In contrast, modelling different total milk yields resulted in large changes in GHGE. The use of top 10 per cent lactations resulted in a reduction in herd size from 127 to 97 for 440-day lactations and a reduction in emissions of 13 per cent.

“This effect was even greater for shorter lactations with herd size being reduced from 129 to 78 cows and a 25 per cent reduction in GHGE for 305-day lactations,” explained Dr Pollott.

He added that the opposite effect was found with the bottom 10 per cent of lactations for milk yield, in other words larger herds and higher GHGE.

Full details: Pollott GE, Wall E and Coffey MP: “Will extended lactations in dairy systems result in a reduction in greenhouse gas emissions?”

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