Repeatability of methane production in cattle fed concentrate and forage diets

Eight Angus steers whose methane emissions had had been found to be higher or lower than predicted when fed a commercial feedlot diet were re-tested on a medium quality forage diet. Methane emissions were within published ranges (136.4 g d-1), but differences in actual vs predicted production between high and low ranked animals were diminished and several animals changed in rankings. This suggests that methane emission characteristics may not persist over time, and that any selection of animals for low methane emission may need to be diet specifi c.


INTRODUCTION
Anthropogenic methane production is a signifi cant contributor to the greenhouse effect, and approximately 12% of this is generated by ruminants (Crutzen et al., 1986).Opportunities for amelioration of methane production by ruminants may include: a. selection for animals which produce relatively less methane, and/or b. dietary manipulation or management of animals' internal environment which predispose them to the production of lower levels of methane.Blaxter and Clapperton (1965) observed that animals produce relatively more methane per unit energy intake on forage rather than concentrate but an important question that has remained unresolved is whether animals that are assessed as high or low CH 4 emitters on one type of diet retain the characteristic or rank on other feed types.
Direct measurement of methane production in respiratory chambers is constrained by the relatively small number of animals that can be screened for methane production.The development of indirect techniques using tracer gases (Johnson et al., 1994) has facilitated the screening of relatively large numbers of animals in the field.This has led to identifi cation of animals that exhibit higher or lower methane production than would be expected based on body size and intake (Pinares-Patiño et al., 2003;Hegarty et al., 2004).
The purpose of this trial was to determine if cattle producing more or less methane than expected demonstrate this characteristic when changed from a concentrate to an exclusively forage diet.

Animal management and feeding
In a preceding study, methane production had been individually measured on 91 Angus steers fed a commercial feedlot diet (12.1MJME/kg DM, 16.0% CP, 100ppm Rumensin ® ).CH 4 production was expressed as a percentage of that predicted by the general equation of Blaxter and Clapperton (1965) and a subset of steers producing more (HIGH emitters; n=6) or less (LOW emitters; n=6) CH 4 than predicted was identifi ed.Emissions from these were measured again on the same feedlot diet (Period 1).The cattle were then depastured for several months.
Eight animals (627±49.2kg) were selected for further study (Period 2) and CH 4 production re-measured, during which the animals were housed individually in adjacent feedlot pens (15 m × 25 m) with unrestricted access to water and chaffed forage sorghum hay (Sorghum bicolour c.v. Superdan; 8.1 MJ ME/kg DM, 11.2% CP).Animals were adapted to the diet for 15 d before measurements were made, with ad libitum feed provided once daily, and refusals collected and weighed.During the measurement period, feed and refusals were sub-sampled daily and analysed (DM, CP, ADF, NDF and ME) at the trial's end.

Methane measurement
Enteric CH 4 production was measured using the sulphur hexafl uoride (SF 6 ) tracer technique (Johnson et al., 1994) as modifi ed by Hegarty et al. (2003).The SF 6 tracer gas was released at a known rate from permeation tubes placed per os in each steer's rumen prior to commencement of gas collection.Expired air was drawn continuously into an evacuated canister fi tted to each animal's back via a capillary tube from tubes placed immediately above each animal's nose (attached to a headstall).Collection took place over 5 sequential, 2 day periods with background samples of CH 4 and SF 6 collected from 2 locations adjacent to the animals' pens.
Methane concentrations were measured using an Innova 1312 multi-gas analyser (Air Tech Instruments), and SF 6 concentrations measured using gas chromatography (Goldsack et al., 1979).Samples having less than 1ppb SF 6 or 30 ppm CH 4 were eliminated from the study.CH 4 emission was calculated by proportion (Johnson et al., 1994) and additionally expressed as a percentage of that predicted by the general equation of Blaxter and Clapperton (1965).

RESULTS
Overall mean CH 4 production in Period 2 was 136.4 gd -1 and did not differ between those steers that were identifi ed as HIGH emitters or LOW emitters in the preliminary screening and in Period 1 (Table 1).HIGH and LOW emitters showed less divergence in CH 4 production than in the earlier two studies and several animals changed from being more than, to less than predicted.methane predicted to be produced from the equation of Blaxter and Clapperton (1965) 2 results of earlier studies (Hegarty et al., 2004) are re-produced for comparison purposes Methane production per unit of feed intake was also higher on the forage diet, increasing from a mean of 1.34 g to 2.26 g CH 4 /MJ ME intake.

DISCUSSION
Between animal variation in CH 4 production has been quantifi ed (Blaxter and Clapperton, 1965;Uylatt et al., 1999) and potential physiological mechanisms underlying genetic differences in ruminant digestion and CH 4 production have recently been reviewed (Hegarty, 2004).The fi ndings in this study that the characteristic of steers identifi ed as HIGH or LOW CH 4 emitters was not maintained across diet types challenges the robustness of genetic selection of low CH 4 emitting cattle.Similar results were observed in sheep (Pinares-Patino et al., 2003) where ranking of animals on the basis of emissions changed with alterations in the composition of diets.Genotype x nutrition interactions are recognized for many livestock production traits, and the results of this study indicate that selection for low methane production in cattle will need to be diet-specifi c.Failure of the general equation used to adequately refl ect methane emissions also suggests that work needs to be done to develop more robust predictors of CH 4 emission in ruminants.

Table 1 .
Actual and predicted methane emissions on concentrate 2 and forage based diets