Methane is a potent greenhouse gas (GHG) emitted from several anthropogenic sources, most notably enteric fermentation in domestic ruminants, primarily during exhalation. To date, however, it is unclear whether the excretion route of methane exhaled by ruminants occurs through the mouth or nostrils and what the pattern of excretion is; this is important in designing equipment and methods to measure the methane emitted by ruminants. Thus, the objective of this experiment was to quantify the exhaled methane excreted by dairy cows via the nostrils and mouth while resting, grazing, and ruminating, as well as the pattern of concentration during and between belching. Four tests were conducted in this study with four dairy cows each. In the first test, resting cows, carrying an electronic spirometry mask (ESM) assembled with a methane sensor in its outlet, were used to measure the concentration of methane in the air expelled through the nostrils and to model the methane concentration between belches; in the second test, a mouth mask was used to measure methane excreted through the mouth; in the third test, an ESM with a methane sensor assembled over the mouth was used to measure methane excreted through the mouth; finally, in the fourth test, a methane sensor was manually placed at 2.0–3.0 cm in front of the mouth of ruminating cows to measure the methane concentration. In the first test, a pattern of methane concentration between belches was identified, as well as a pattern of the methane concentration and volume of air exhaled during belching. Methane excretion through the mouth was not detected in any of the tests; hence, it is concluded that dairy cows, while resting, ruminating, and grazing, emit enteric methane only through the nostrils under normal respiration conditions. This is important in the respiratory physiology of ruminants and in designing equipment and methodology to measure cranial methane excretion.

Dual-purpose livestock farming in the Colombian low tropics faces nutritional limitations that affect productivity and increase enteric methane (CH₄) emissions. Therefore, it is necessary to evaluate supplementation strategies that optimize the use of nutrients and reduce environmental impacts. The objective of this study was to evaluate the effects of three types of dietary supplementation—mineralized salt (MS), proteinized salt (PS), and concentrated feed (CO)—on milk production, efficiency in the deposition of dairy nutrients, and enteric CH₄ emissions in dual-purpose grazing cows in the Colombian Amazon. Twenty-four F1 Holstein × Gyr cows were used in a completely randomized block design. Nutrient intake, digestibility, composition, and efficiency in the milk deposition of Ca, P, K, and N were measured; additionally, CH₄ emissions were assessed using a portable electronic spirometry mask. The results indicated that the treatments did not modify the total intake of dry matter or the production and composition of milk. CO and MS supplementation improved the efficiency of Ca and P (p < 0.03) deposition in milk, while no differences were observed for N and K. CH₄ emissions (196 ± 17.9 g/cow/day) were not affected by the supplements and were within the ranges reported for dual-purpose tropical systems. However, with the supplementation with CO was a lower losses of gross energy intake as CH₄ (Y_m) (p < 0.05). In conclusion, supplementation with the evaluated products did not impact milk production or CH₄ emissions, although with CO was lower energy losses as CH₄ and, CO and MS improved the use of Ca and P, which highlights the importance of adjusting supplementation to optimize nutritional efficiency without increasing greenhouse gas emissions.