Reducing animal numbers may seem an obvious way to reduce FCH4. However, equally obvious, this method may not be acceptable to many farmers if it threatened their livelihood. Moreover, while animal numbers have been reducing in some countries (for example European Union and US), the global ruminant population has increased slightly over the past ten years according to FAO (2009). This reflected greater demand for ruminant animal products, milk and meat (Steinfeld et al, 2006). Demand has increased more for meat, which has been increasingly supplied by the production of monogastric animals like pigs and chickens (Galloway et al, 2007). In addition, there are plant-based alternatives for producing human dietary protein. However, utilization of these production systems has not been fully realized, especially for economically advanced and advancing societies, though change might be possible according to Smil's (2002) insightful and constructive analysis. Thus, animal numbers should not be considered in isolation from production.
Farmers have always sought improvement in production efficiency. This may be defined on the basis of feed intake or ME requirement. With this definition, efficiency may be related to a partitioning of feed intake into MEm and production requirements. The MEm and associated FCH4 may be considered fixed, depending on the ruminant's weight. Thus, the additional ME requirement and FCH4 will be determined by the production rate. This means an increase in feed intake and thus production corresponds with decreases in the proportion of FCH4 attributable to MEm and FCH4 per unit product. Consequently, for a given quantity of product, the farmer can reduce FCH4 by utilizing the most productive ruminants. For a static production level, this argument has been interpreted to suggest an FCH4 mitigation strategy based on ruminant selection. However, a farm's production level is unlikely to be static, so reducing FCH4 per unit product does not necessarily correspond with reduced F.
To illustrate the relationship between production efficiency and FCH4, we have analysed dairy production for cows fed by grazing fresh pasture. For pasture in New Zealand, the GE content was 18.4MJ GE kg-1 DM (Judd et al, 1999). The GE content of CH4 is 55.6MJ kg-1. Combining these values with a mean value for variable m of 6 per cent and converting the units, the mean m was 20g CH4 kg-1 DM. For context, this calculated value was 14 per cent less than the mean value based on meta-analysis of the sheep (fed grass) measurements discussed earlier. For a grazing, non-lactating cow with constant w = 450kg, daily DMI was 5kg according to Clark et al (2005). Based on this DMI and the mean m, the daily FCH4 was 100g. For argument, we attributed this DMI and FCH4 to MEm. When this cow's daily DMI was 10kg and she was lactating, daily milk production was 12kg (Clark et al, 2005). The corresponding daily FCH4 was 200g, so half was attributable to production and there was 17g CH4 kg-1 milk. Increasing her daily DMI to 14kg, daily milk production was 24kg (Clark et al, 2005). Now, the corresponding daily FCH4
was 280g, so one third was attributable to production and there was 12g CH4 kg-1 milk. Thus, although doubling the milk production corresponded with a 29 per cent decrease of FCH4 per unit product, FCH4 itself had increased by 40 per cent.
Diet manipulation has been considered to present other opportunities to reduce FCH4 from ruminants. For example, increasing the proportion of grain has been shown to reduce FCH4 (Lovett et al, 2006). While increased profitability was also reported, circumstances can be different or change, so this cannot yet be considered a general recommendation. Adding lipids to the diet has also significantly reduced FCH4 according to Beauchemin et al (2008). However, 40-55 per cent of the global FCH4 from ruminants may be attributed to grazing animals, outdoors for most of their lives and consuming forage diets (Clark et al, 2005). The type of forage grown can be changed to legumes, for example, and an increased concentration of condensed tannins has been shown to reduce FCH4 of grazing ruminants (Waghorn and Woodward, 2006). However, alternative species are unlikely to be grown alone, so they must be able to grow with, and not be overwhelmed by, the species currently utilized (for example ryegrass).
Modification of the rumen can involve the administration of chemicals. For example, use of the ionophore, monensin, has reduced FCH4 in some circumstances (Beauchemin et al, 2008). Monensin also conveyed productivity and health benefits, so it has attracted widespread scientific attention. However, the rumen microbial community tends to be very adaptive (McAllister and Newbold, 2008), so the reduced FCH4 may not be long lasting. These compounds have been classed as antibiotics and their use may be unacceptable to some ruminant product customers and illegal in some production locations. There have been other compounds with claimed efficacy for reducing FCH4 that can broadly be classed as rumen modifying agents; these include yeasts, condensed tannin extracts, probiotics and enzyme-based feed additives. Although available commercially, we have found data supporting the claimed efficacy to be sparse and further evidence would be needed before any of these products could be recommended to farmers on this basis.
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