Quantification and mitigation of enteric methane emissions from grazing Jersey cows

Abstract

Increasing evidence for global warming has amplified the need to accurately verify national greenhouse gas (GHG) inventories, and to validate on-farm GHG mitigation strategies. Agriculture is known to significantly contribute to GHG with ruminants identified as the single most important source of anthropogenic methane (CH4) emissions – a potent GHG. Several CH4 mitigation strategies have been developed, but few succeeded in terms of instant results, efficacy, persistency and practicality, of which all are vital for the adoptability thereof on farm level. Concentrate feeding level and dietary nitrate addition have been identified as CH4 mitigation strategies that are most likely to be adopted on farm level. Although the latter CH4 mitigation strategies have been extensively evaluated on cattle in confinement, limited research exist on the effect thereof on pasture-based dairy cattle across seasons. This study aimed to directly measure CH4 emissions from pasture-based Jersey cows grazing pasture, while determining the effect of concentrate feeding level and dietary nitrate addition as CH4 mitigation strategies on CH4 emissions, cow performance and rumen fermentation. Additionally, this study aimed to reduce or eliminate animal skin abrasions imposed by current back-mounted harnesses facilitating the SF6 technique, and to compare CH4 data derived from the SF6 technique with that of a short-term measurement technique. This study consisted of six trials. The first trial investigated the effect of concentrate feeding level (0, 4, and 8 kg/cow per day; as fed basis) on enteric CH4 emissions, cow production performance and rumen fermentation of dairy cows grazing kikuyu (Pennisetum clandestinum) dominant summer pasture. Sixty multiparous Jersey cows (nine rumen-cannulated) were used in a randomised complete block design with the cannulated cows in a 3 × 3 Latin square design. Total dry matter intake (DMI), milk yield, energy-corrected milk (ECM), and milk lactose content increased linearly, while pasture DMI decreased linearly with increasing concentrate feeding level. Methane production (323 to 378 g/d) increased linearly, while CH4 yield and CH4 intensity (milk yield and ECM) decreased linearly with increasing concentrate feeding level. Diurnal ruminal pH and in sacco dry matter (DM) and neutral detergent fibre (NDF) disappearance decreased linearly with increasing concentrate feeding level. Accetic and propionic acid were unaffected by treatment, while butyric acid increased linearly and quadratically with increasing concentrate feeding level. It was concluded that supplementing a high concentrate feeding level on pasture-only diets increases enteric CH4 production by 17% but reduces CH4 emissions per unit of DMI by 14% and per unit of milk yield and ECM by 39% and 41%, respectively, from cows grazing kikuyu-dominant pasture during summer. The second trial was in essence a repeat of the first trial, but on ryegrass (Lolium perenne) dominant pasture to determine whether the CH4 mitigation efficacy of the concentrate feeding level was influenced by seasonal change in pasture composition. Sixty multiparous Jersey cows (six rumen-cannulated) were used. Total DMI, milk yield, ECM, milk lactose content and pasture DMI response were similar to the previous trial, however in this trial milk fat content decreased with increasing concentrate feeding level. Volatile fatty acid concentrations and ruminal pH were mostly unaffected by treatment, while DM disappearance decreased and NH3-N concentration increased with increasing concentrate feeding level. Methane production (258 to 302 g/d) and CH4 yield were unchanged, while CH4 intensity decreased linearly with increasing concentrate feeding level. It was concluded that concentrate supplementation on high quality pasture-only diets have the potential to effectively reduce CH4 emissions per unit of milk yield by 20% from cows grazing perennial ryegrass-dominant pasture during spring. The third trial investigated the effect of dietary nitrate addition (0, 11, and 23 g of nitrate/kg of DM; control, low nitrate, and high nitrate, respectively) on enteric CH4 emissions, cow production performance and rumen fermentation of dairy cows grazing kikuyu-dominant pasture containing approximately 3 g of nitrate/kg of DM. Fifty-four multiparous Jersey cows (six rumen-cannulated cows) were used in a randomised complete block design with the cannulated cows in a 3 × 3 Latin square design. Concentrate was fed at 5.4 kg of DM/cow per d and formulated to be isonitrogenous by substituting urea. Cows were gradually adapted to concentrates over a 3-wk period. Although total DMI was unchanged, the high nitrate diet decreased concentrate DMI and milk yield but increased pasture DMI. Daily CH4 production (313 to 280 g/d), CH4 yield (21.8 to 18.7 g/kg of DMI) and CH4 energy per gross energy intake (Ym; 6.9 to 5.9%) tended to decrease linearly with increasing dietary nitrate addition. It was concluded that dietary nitrate fed to grazing dairy cows showed some promise as CH4 mitigation strategy. Furthermore, rumen fermentation was not adversely affected; however when feeding high levels of nitrate a decrease in milk yield could be expected due to a decrease in concentrate DMI. The fourth trial was a repeat of the third trial, but on perennial ryegrass-dominant pasture and with only two treatments with essentially a different experimental design. The high nitrate treatment in the third trial was not repeated due to the observed partial refusal of concentrate and decreased milk yield. Thirty-two intact and eight rumen-cannulated multiparous Jersey cows were subjected to a replicated 2 × 2 Latin square design supplemented with one of two concentrates containing either urea (urea treatment), or urea and nitrate (nitrate treatment; containing 0.3 and 15.2 g of nitrate/kg of DM, respectively). Grazed pasture contained approximately 7.3 g of nitrate/kg of DM. Total DMI, milk yield, CH4 production (400 and 405 g/d) and most rumen fermentation parameters were unaffected by treatment. Dietary nitrate increased milk components (except for milk protein content). Minor effects on ruminal pH were observed with an increasing tendency towards the nitrate group. It was concluded that although dietary nitrate supplementation is not an effective CH4 mitigation strategy for dairy cows grazing perennial ryegrass, increases in milk fat and lactose content may be expected. The fifth trial focused on improving the back-mounted harness of the SF6 technique throughout the four main trials in terms of minimising skin abrasions and lesions imposed by the harness. In conclusion, a cost-effective back-mounted harness for grazing dairy cows that facilitates the SF6 technique for measurement of enteric methane emissions while not causing any skin lesions was developed. The sixth trial was a comparison study between CH4 emission rates as measured by the LMD and SF6 technique from lactating dairy cows grazing perennial ryegrass-dominant pasture. Methane production was determined from six lactating Jersey cows on pasture using both techniques. Methane output data from the LMD had a higher (0.6 vs. 0.4) between-cow coefficient of variation compared with data obtained from the SF6 technique. Methane production as measured by the SF6 technique (348 g/d) was higher compared with the LMD technique (82.6 g/d). Findings of this study indicate that there is a need to improve the LMD operating protocol and scale-up factors to accurately convert CH4 concentration (ppmv.m) to CH4 production (g/d). In conclusion, this research has provided an understanding of the use and potential of concentrate supplementation as CH4 mitigation strategy for dairy cows in pasture-based systems, and has shown that dietary nitrate has the potential as CH4 mitigation strategy for dairy cows in pasture-based systems. Furthermore, this research has provided a novel back-mounted harness for grazing dairy cows in facilitating the SF6 technique in enteric CH4 measurement.