Comparison of enteric methane emissions from Holstein Friesian cattle grazing on single species sward or mixed-species sward pastures.

Since the nineties, a combination of fluctuating and extreme prices of inorganic N fertilisers, combined with restrictions on N fertiliser rates per hectare per annum in certain countries, and societal pressure to reduce environmental impacts, have combined to create pressure on farmers to consider the reintroduction of functional diversity to their pastures. The most common example of this is the addition of clover to perennial rye grass (PRG), creating a binary mixture. Somewhat later, research suggested that further increases in diversity were beneficial, leading to the concept of targeted multispecies swards becoming a topic of scientific interest in the 2000s. Several studies since showed that multispecies can increase yield, partly or entirely replacing the need for N fertilisation, thus creating a cheaper and more self-reliant way of intensifying grassland productivity. The multispecies swards most often represented are perennial grasses, legumes, and herbs (forbs), providing possible benefits of being more productive, resilient, and environmentally friendly. In the latter context, including additional plant species to monoculture grazing swards has also been proposed as a means to lower methane emissions. To test this hypothesis, the objective of the study cited was to compare enteric methane production from lactating dairy cows grazing either PRG or a multi-species sward containing forage legumes and herb species (MSS).

Thirty-six lactating Holstein cows (226 ± 23 days in milk and producing 22.7 ± 4.11 kg milk per day) were paired according to milk yield and allocated either the PRG or MSS treatments. Initially, an adaptation period of 28 days allowed the cows to adapt to the grazing and be trained for use of the Pasture GreenFeed methane measuring equipment, before being separated into treatment groups to strip graze the corresponding sward types. The cows were milked twice daily and received a fixed allowance of 2 kg concentrate mix per day, fed during milking, and they also had access to a TMR for approximately a half hour per day. When on pasture they had continuous access to the GreenFeed equipment. Milk yield was recorded daily and the composition measured weekly, allowing calculation of methane intensity (g per FPCM).

The results showed that rate of methane emission was higher for MSS than PRG (397 vs 363 g per day; P = 0.05). The milk yield was also higher (27.6 vs 24.8 kg per day; P = 0.03), but there was no difference in milk composition. Consequently, there was no treatment difference in methane intensity (13.6 vs 13.3 g per kg FPCM; P = 0.73) between treatments.

It was concluded that the increased rates of methane and milk production by the cows grazing MSS might have been due to higher feed intake, which unfortunately could not be measured. Although promising with regard to milk yield, other studies have shown that results within and between different levels of species richness are not always consistent, suggesting that not all multispecies swards will perform alike, as a multitude of other factors interact to determine the success of one multispecies sward over another. These include soil type and fertility, species choice, species group proportions, sward management under either grazing or cutting, fertiliser regimes, and grazing management practices. In terms of the hypothesis tested here though, there was no difference between PRG and MSS in methane intensity, i.e. methane production per kg FPCM, suggesting no major difference in enteric fermentation.