The effect of increasing sward species diversity on enteric methane emissions from Holstein-Friesian and Holstein-Friesian × Jersey crossbred dairy cows in a rotational grazing system.

In many pasture-based milk production systems, simple and productive forage systems have resulted in swards dominated by perennial ryegrass (Lolium perenne L.), as they show high productivity and nutritional value over a comparatively long growing season (Baker et al., 2023). However, the continued use of this system is now often questioned, due to the high reliance on artificial N fertilizer, which may increase GHG emissions and the risk of nutrient losses to air and water. One alternative is the inclusion of legumes such as white clover (Trifolium repens L.) into the grazing sward which should reduce artificial N fertilizer requirements, and therefore reduce N2O emissions. Clover inclusion may also reduce methane yield compared with grass-only swards, but there are also results to the contrary.

A second alternative is the inclusion of pasture species or “herbs,” such as plantain (Plantago lanceolata L.) and chicory (Chicorium intybus L.), in grass-clover swards, which recently showed environmental benefits such as improved drought tolerance, reduced urinary nitrogen excretion and reduced nitrate leaching. The inclusion of such a multispecies sward may also potentially aid the reduction in carbon emissions by offsetting the methane produced by the grazing or feeding of the sward to dairy cows, as some studies comparing swards dominated by herbs in grass or grass and white clover have shown reduced methane output from herbal swards. However, there have also been conflicting findings with some studies showing similar and sometimes greater methane emissions for multispecies. Consequently, the authors of the study cited aimed to investigate further.

In the study, an experiment was conducted to investigate the effect of sward system and dairy cow breed on enteric methane emissions from spring-calving grazing dairy cows using GreenFeed technology (C-lock Inc.). The experiment comprised three sward systems: a perennial ryegrass (Lolium perenne L.) monoculture receiving 250 kg N per ha per year (PRG), a perennial ryegrass white clover (Trifolium repens L.) sward receiving 125 kg N per ha per year (PRGWC), and a multispecies sward sown with grasses, legumes and herbs receiving 125 kg N per ha per year (MSS). Each sward system had its own herd of dairy cows on a total area of 18.7 ha divided into 20 paddocks. Each herd comprised Holstein-Friesian (HF) purebred and HF × Jersey crossbred (JFX) animals, which were divided equally across each sward system. Milk production and methane emissions were measured from mid-May to mid-October, and dry matter intake (DMI) and rumen characteristics were measured in late-July and early-October (165 and 228 days in milk [DIM], respectively).

The results showed that milk solids (milk fat + protein) was greatest for cows grazing MSS due to an associated increase in DMI. The greatest average daily methane output across the study was observed for cows grazing PRGWC (311 g per day) while those grazing PRG and MSS were similar (294 and 297 g per day, respectively). Methane intensity (g methane per kg milk solids) was reduced in the MSS cows compared with both the PRG and PRGWC cows (208, compared with 217, and 219 g per kg milk solids respectively). The lowest methane yield (methane per kg of DMI) was also observed for the MSS cows (15.5 g per kg), whereas there was no difference between the PRG and PRGWC cows (17.6 and 17.7 g per kg, respectively). Greater VFA concentrations were observed for both the MSS and PRG cows compared with the PRGWC cows, whereas the MSS cows had a greater butyrate proportion compared with the cows grazing the other swards.

Breed also had a significant effect on both milk and methane production characteristics with JFX animals having increased fat and protein concentration and increased milk solids production compared with HF. The JFX cows also had reduced BW. However, breed had no effect on DMI or methane yield, but JFX had improved feed efficiency, both per kilogram of DMI and per kilogram body weight. The HF cows had an increased methane intensity (223 g per kg milk solids) compared with JFX (207 g per kg milk solids), whereas in JFX the rumen ammonia concentration and the VFA acetate-to-propionate ratio were increased, the propionate proportion decreased, and the butyrate proportions increased compared with HF.

Summary: Cows grazing PRGWC had similar methane yields as cows grazing PRG, although the cows grazing PRGWC had increased total enteric methane output due to increased DMI. The MSS sward further increased DMI and milk production, while simultaneously reducing methane yield compared with PRG and PRGWC. This ultimately resulted in the cows grazing MSS having reduced methane intensity compared with PRG and PRGWC. This study also found that JFX cows had a reduced daily methane output and intensity compared with HF cows.

In conclusion: The results of the study highlight the potential for more diverse swards and Holstein-Friesian × Jersey crossbreeding to reduce enteric methane emission intensity within pasture-based dairy systems.