Review: Selecting for improved feed efficiency and reduced methane emissions in dairy cattle.

It is generally accepted that by selecting for feed efficient cows, enteric methane emissions should be reduced, but there is currently not consensus on their relationship. Limited evidence does indicate that residual feed intake (RFI) is favourably correlated with enteric methane emissions throughout lactation. RFI represents the deviation of individual cow feed intakes from the expected relationship between feed intake (energy requirements) and milk production, and from an efficiency point of view, one would want to select the cow that has lower feed intake for a given milk production. Logically then, since enteric methane is a function of the feed entering the rumen, the cow which eats less will also produce less methane. The difficulty arises to record feed efficiency and methane emission traits in genetic improvement programmes as measurements are complex, costly and time-consuming. One challenge is the lactation curve per se: since the relationship between milk production and feed intake and feed composition (and therefore methane production) continuously changes during lactation, it is difficult to establish quantitative relationships. This implies that the ranking of animals for feed efficiency and methane emission traits can differ depending upon the type and duration of measurement used, the trait definitions and calculations used, the period in lactation examined and the production system (reflecting feed composition), as well as the interaction among these factors. The results of some trials are summarized below.

One interesting trait is rumination time (RT), which can be measured relatively easy. RT is a function of the amount of rumen fermentation and should therefore be correlated with the end-products of fermentation, including methane. In a large scale trial, the genetic relationships among RT, methane production (MeP) and milk production traits were studied. The estimated heritabilities were moderate for RT (0.45 ± 0.14), MeP (0.36 ± 0.12), milk yield (0.40 ± 0.08), fat yield (0.29 ± 0.06), protein yield (0.32 ± 0.07), and energy-corrected milk (0.28 ± 0.07). A favourable negative genetic correlation was estimated between RT and MeP (−0.53 ± 0.24), whereas a positive favourable genetic correlation was estimated between RT and energy-corrected milk (0.49 ± 0.11). These results indicate that RT is genetically associated with MeP and milk production traits, but the high standard errors illustrate the difficulties discussed above. RT will also be associated with fibre digestion in the rumen. As nutrition studies indicate that selecting low emitting animals may result in reduced efficiency of cell wall digestion (NDF), this should reflect in lower RT as in these cows the indications are that their digestion rate and passage rate are probably faster, implicating a slightly different rumen microbiome and more absorption of nutrients from the lower digestive tract than the average cows.

Another study aimed to estimate the correlations between residual feed intake (RFI) and methane emissions expressed in g per day methane production (MeP), g per kg of fat- and protein-corrected milk methane intensity (MeI), or g per kg of DM intake methane yield (MeY) throughout lactation. Low correlations were found between RFI and MeY, which vary from positive to negative, ranging from -0.18 to 0.17. Both MeP and MeI were favourably correlated with RFI, as was MeY during the first half of lactation. These correlations are mostly favourable for genetic selection, but confirmation of these results is needed with genetic correlations over larger datasets.

In conclusion: Despite measuring and interaction difficulties, there is sufficient evidence to show that selection for feed efficiency either via RFI (which requires individual feeding facilities) or by other means, will lower enteric methane production. In doing so benefits will accrue both economically and in reducing the carbon footprint of the dairy farm. To improve feed efficiency, the author previously suggested using of the following formula which is independent of feed intake measurements:

GFE (kg/kg) = 1.881 + 1.344BFY −0.003LW, R² = 0.91.                                                                                                                            

where: GFE = gross feed efficiency, BFY = butterfat yield (milk fat % x milk yield), and LW = live weight.

The relationship of GFE with butterfat is explained by better energy mobilization in more efficient cows. Utilizing this equation in selection will enhance both milk fat and milk yield which is beneficial. However, unabated selection for milk fat % could be negative to protein % and should be controlled. Of further significance in the equation is that LW is negatively associated with GFE, which means that GFE declines with LW. Farmers should therefore put a ceiling on cow LW to maintain GFE and limit maintenance needs of the cow. Finally, for farmers that have the equipment, measuring RT in conjunction with milk yield to identify those cows with excellent milk production and lower than average RT, should have additional advantages towards the quest for a more efficient herd.