Table 1.

Methods for measuring CH₄ emissions.

Method	Short Elucidation
Respiration chambers (RC)	There are two types of RC: closed-circuit and open-circuit [25]. While closed-circuit systems are practically never used nowadays, open-circuit chambers are the most often exploited type, with varying degrees of complexity [25,47,48]. Individual animals are typically kept in chambers between 2 to 7 days, and CH4 emissions are estimated based on gas flow and changes in gas concentrations between the air coming in and out [10,25,48,49,50]. The chamber approach is expensive in terms of both investment and labor, and it has been accused of affecting feeding behavior. In trials employing transparent chambers, however, no impacts on dry matter intake (DMI) were observed [49]. However, only one cow may be tested at a time, and each test requires several hours in the respiration chamber, limiting research efficiency [10,18,25]. Almost in all studies, a single gas analyzer was used to measure in and out methane concentrations, often for two or more chambers [10,18,42]. When Garnsworthy et al. compared several different methods of GHG measuring, it was found that respiration chambers were the most accurate method. None of the correlations with other methods exceeded 0.90 [10].
Sulfur hexafluoride (SF6) tracer technique	The method is invasive—the cow must consume a bolus carrying the tracer, and the SF6 tracer is an exceptionally potent greenhouse gas [18,42]. The underlying premise is that the rate of SF6 gas release from the rumen is calculated in order to calculate the CH4 emission measurement [25,51]. The approach is suited for enclosed and free-roaming animals, and it involves inserting a permeation tube with a known SF6 gas release rate into the animal’s reticulorumen [49,52]. A tube hooked to a halter and connected to an evacuated canister worn around the animal’s neck or back is the basic premise behind this technique. Near the animal’s nostrils, the air is taken for testing purposes. It takes roughly 24 h for the canister to fill up between 50% and 70% because of an airflow restriction provided by a capillary tube. Methane emission rates are calculated by multiplying the predetermined SF6 release rate by the canister methane to SF6 concentration ratio [10,25]. The technique is more beneficial when evaluating CH4 emissions from individual animals. Wearing the device and daily handling to change canisters may affect animal behavior and feed intake. The sulfur hexafluoride (SF6) tracer technique is far less intrusive than breathing chambers because cows remain in the herd [42].
Spot sampling technique/ Gas-flux quantification system	The cornerstone of spot sampling approaches is the collection of acceptable short-term breath data for emission measurements. The techniques employ spot measurements of exhaled CH4 during milking or feeding. These procedures are typically automated, noninvasive, and non-intrusive, allowing for a high rate of animal throughput [45,49]. Breath sampling is taken during milking and feeding. The feed bin could be at a concentrate feeding station or an autonomous milking station [18,52,53]. These methods are referred to as “sniffer methods” since they use devices originally developed to detect harmful gas leaks. Air is sampled near the animal’s nostrils using a tube attached to a feed bin and immediately connected to a gas analyzer [10,18]. Methane concentrations measured during a sample visit of 3 to 10 min can be expressed as the overall mean or the mean of eructation peaks. Breath-sampling approaches provide substantial advantages over other methods for large-scale measurement of methane emissions by individual animals [20,25]. Some patented methods, such as the GreenFeed system, work in the same method as sniffer methods [25]. The GF method is based on the idea that a single animal’s daily average CH4 emission can be estimated by combining multiple short-term Methane emission measurements obtained throughout the day [25]. Breath-sampling procedures are noninvasive because animals are unaware of the apparatus and are in their natural environment after it is deployed. Animals continue their normal schedule, which includes milking and feeding; therefore, no animal training, handling, or dietary changes are required. Although more sophisticated gas analyzers are available, the equipment is quite inexpensive, and the operating costs are insignificant [20,45].
Carbon dioxide as a tracer to estimate daily methane emission	The methane and carbon dioxide ratio technique predicts CH4 output by certain species by forecasting carbon emissions and sensing methane and carbon dioxide concentrations [18,53]. This method demands knowledge about the ration’s consumption, energy content, and heat increase [54]. Using the CO2 technique does not consider the difference in CH4 emissions between efficient and inefficient cows; according to Huhtanen et al., Researchers found a strong correlation between the efficiency of low and high-efficiency cows. The technique overestimated the amount of CH4 produced by productive cows while underestimating the amount produced by ineffective cows [55]. Because it is so easy to apply to many animals, the standard error of means can be reduced [25].
Infrared ray spectroscopy, laser technique	Lasers have long been applied for gas detection in environmental monitoring, air quality monitoring, security, and health care [25]. Hand-held gas detectors for remote measurements of column density for methane-containing gases. It is based on infrared (IR) absorption spectroscopy. It uses a collimated semiconductor laser as an excitation source and wavelength modulation spectroscopy’s second harmonic detection to establish a methane concentration measurement [25,56]. Methane concentration measurements are performed manually using a portable instrument around 1–3 m away from the animal. The data acquisition sequence comprises small spans of 2–4 min. The resulting data is a series of peaks representing the animal’s breathing cycle [56,57]. The laser methane detector (LMD) can be used in the animal’s natural environment; however, a constraint is required during the measuring process to ensure accuracy. Results can be affected by factors such as the distance from the animal, the angle of pointing, the animal’s motion and moving direction, the airflow movement, and temperature in the barn since the LMD measures methane in the plume issuing from the animal’s nostrils [58].
Face mask (FM) method	The method for spot samplings of respiratory exchange and CH4 emissions is based on animals trained to remain in sternal recumbency for 30 min measurement periods taken every 2–3 h, with up to 7 measures per day [25]. In terms of assessing gas exchange and changes in the exhaled CH4 concentration, the basis of this method is identical to that of RC. It consists of a mass flow controller, gas sampling unit, and CH4 emission analyzer attached to each face mask. Gas readings are corrected for humidity, lag time, drift, and CH4 emission (mL/min) changes for each period [25,59]. The FM approach is less expensive and simpler than SF6 or RC. Its mobility allows it to measure multiple areas in order to collect CH4 emissions [25,60].