How do grass species and harvesting impact milk and methane?

Cows consuming timothy harvested 3 times per year had greater organic matter digestibility, greater energy-corrected milk yield, and lower methane yield. Photo: Michel Velderman

As the dairy industry continues the fight to reduce its contribution to GHG emissions, a focus on nutritional strategies remains most relevant. As grass silages are common in the dairy cow’s diet, researchers from the Norwegian University of Life Sciences investigated how different grass species and harvesting frequency impact milk and methane production. Their findings offer a simple yet practical tool to reduce methane production while maintaining milk yields.

Reports show that in North America, northern and western Europe, silages based on grass and grass-clover mixtures constitute a large part of dairy cow diets. Within a 3-cut system, the herbage is often harvested earlier (i.e., plants harvested at the vegetative stage) compared to silages produced within a 2-cut system (i.e., plants harvested at a more mature stage in the first cut). Previous research work shows that harvesting at a more vegetative stage promotes greater organic matter digestibility and dry matter intake of silages due to lower ash-corrected neutral detergent fibre organic matter and indigestible fibre concentrations.

Some reports also show that frequent harvesting may alter ruminal volatile fatty acids concentrations toward less acetate and butyrate and more towards propionate, reducing hydrogen (H₂) availability for methane formation. However, reports on the relationship between feeding silages prepared from grass harvested at different maturity stages are inconsistent; this was the basis of the current study published in the Journal of Dairy Science.

Grass silages

In this study, researchers used Norwegian Red dairy cows to evaluate how silages from different grassland species (timothy, perennial ryegrass and red clover) and harvesting frequencies impact dairy cows’ feed intake, milk production, nutrient digestibility and methane (CH4) emissions. Cows were offered the silages ad libitum supplemented with a fixed level of concentrate.

The grass silage-based diets were as follows:

Timothy (Phleum pratense L.) in a 3-cut system (T3)
Timothy in a 2-cut system (T2)
Perennial ryegrass (Lolium perenne L.) in a 3-cut system (PR3)
Red clover (Trifolium pratense L.) in a 3-cut system (RC3)
A mix of T3 and RC3 (50:50 on DM basis) (T3/RC3)

Feed, milk and feed

Looking at timothy grass silage (T3 vs T2), there were no differences in dry matter intake, however, the lower ash-corrected neutral detergent fibre organic matter concentration and greater organic matter digestibility resulted in, as expected, greater energy-corrected milk production for T3 compared with T2. Higher organic matter digestibility was expected as in young and less mature grass, the plant cell wall and lignin concentrations are lower.

The organic matter digestibility of T3 did not differ from PR3, but the T3 cows produced more milk (energy-corrected milk) when fed T3. This was attributed to the higher digestibility of ash-corrected neutral detergent fibre organic matter and a greater dry matter intake of T3 compared to PR3. Dry matter intake and energy-corrected milk yield were lowest for RC3, which aligned with the lower dry matter concentration in RC3 and the observed decrease in digestibility of both organic matter and ash-corrected neutral detergent fibre organic matter with an increasing proportion of red clover in the diet.

The mix ‘T3/RC3’ diet had approximately 14% greater ash-corrected neutral detergent fibre organic matter digestibility than the red clover (RC3) diet, explaining the increase in silage dry matter intake and energy-corrected milk yield in the T3/RC3 diet (Table 1).

Although the red clover (RC3) diet contained more crude protein than the timothy (T3) diet, energy-corrected milk yield was higher in the T3-fed cows. This corresponded to a lower net energy for lactation observed in the red clover (RC3) diet, suggesting a protein-energy imbalance in the rumen. The researchers concluded that the main factors explaining the differences in energy-corrected milk yield are feed intake (dry matter intake) and nutrient digestibility (Table 2).

Methane emission

The increase in organic matter digestibility led to increased energy-corrected milk production and hence a lower methane (CH4) emission intensity in Timothy harvested 3 times (T3) compared with 2 times per year. The researchers suggested that the 8-percentage-point increase in organic matter digestibility from T2 to T3 in the present study was not sufficient to increase the CH4 production.

According to the researchers, the increased CH4 yield (g/kg digestible organic matter) and intensity observed in the RC3 diet were related to the low digestibility of both ash-corrected neutral detergent fibre organic matter and organic matter (Table 2).

“The low digestibility may have led to unfavourable conditions for microbial synthesis and a surplus of ammonia in the rumen. This probably lowered dry matter intake and energy-corrected milk yield, corresponding to an increase in CH4 yield and intensity,” they said.

Conclusion

The researchers concluded that, despite the similar dry matter intake, cows consuming timothy harvested 3 times per year had greater organic matter digestibility, greater energy-corrected milk yield, and lower methane yield and intensity than cows consuming timothy harvested 2 times per year. Replacing timothy with perennial ryegrass increased methane and intensity while increasing the inclusion rate of red clover from 0% to 100% linearly increased methane production, yield, and intensity.

“As farmers are being directed to reduce enteric methane emissions, our study shows that enteric methane emissions in dairy cows can be mitigated by increasing harvest frequency and using timothy grass rather than perennial ryegrass and pure red clover silage in the diet,” said the researchers.

This article was based on the publication ‘Milk production and methane emissions from dairy cows fed silages from different grassland species and harvesting frequencies’, Journal of Dairy Science, 2024.