Los modelos de incubación

The cause of the reduction in CH4 yield with increasing feed intake has not been clearly

defined, although most suggestions are that higher feed intakes result in shorter rumen residence times, with lower digestion, compared to low feed intakes (Ulyatt et al., 1984, Johnson et al., 1993, Dewhurst et al., 2003, Pinares-Patiño et al., 2003b, Pinares-Patiño

et al., 2007).

With an increase in feed intake there is more substrate entering the rumen, an increase in rumen digesta load (Ulyatt et al., 1986, Waghorn et al., 1986, Pinares-Patiño et al., 2003c), and more substrate is available for microbial colonisation and H2 generation

(Hegarty et al., 2007b). The mean retention time (MRT) of feed can be influenced by the rumen digesta load, which changes in response to feed intake, and is affected by the physical capacity of the rumen in individual sheep. A short MRT is associated with rapid digestion or outflow from the rumen, and is characteristic of high feed intakes or readily digestible DM. For example, Thornton and Minson (1972) fed sheep lucerne chaff and found that as intakes increased from 607 to 1180 g/d, MRT decreased from 26.7 to 14.5 h, respectively. A short MRT may also be associated with a low CH4 yield

from the diet. A longer MRT is generally associated with low intakes or more fibrous diets, which provide a greater opportunity for digestion and CH4 production. In this

study, sheep fed at low intakes (e.g. 0.40 kg/d), had a similar digestibility, but a higher CH4 yield, compared to sheep at high intakes (e.g. 1.60 kg/d). Although not measured

CHAPTER 6: Effect of feed intake on methane emissions from sheep 147 interaction between digesta load, residence times and passage rates, and this requires further investigation.

Faster passage rates (i.e. a decreased MRT of digesta in the rumen), characteristic of high feed intakes, are thought to result in a shift in fermentation pathways towards more propionate production and consequently less CH4 formed per unit of feed eaten (i.e.

CH4 yield) (Dewhurst et al., 2009, Janssen, 2010). When feed intakes were set from

0.40 up to 1.60 kg/d, molar percentages of propionate were predicted to increase as CH4

yield decreased (Table 6.3). Regression analysis of data from all four experiments found the molar percentage of propionate predicted up to 60% of the variation in CH4

yield.

Based on evaluations of CH4 yield in relation to feed intake (e.g. Blaxter and

Clapperton, (1965), Johnson et al., (1993), Yan et al., (2000, 2010), Beauchemin and McGinn, (2006), Molano and Clark, (2008), Muetzel et al., (2009), Sauvant and Giger- Reverdin, (2009), Chapter 4), it is established that there is a decrease in CH4 yield as

feed intakes increase. However, this relationship does vary and is less evident for roughages compared to concentrate diets. Blaxter and Clapperton (1965) showed that the reduction in CH4-E/GEI for each multiple of MEm increase in intake, was greater for

pelleted diets (0.021) compared to roughages (0.008). Increasing intakes of concentrate diets from 1 to 2 x MEm reduced CH4-E/GEI by 0.016 in cattle (Johnson and Johnson,

1995) and 0.015 in sheep (Moss et al., 1995). However, when cattle were fed fresh grass or silage diets, the effect of intake on CH4-E/GEI was less (about 0.008) (Yan et

al., 2000, Beauchemin and McGinn, 2006, Yan et al., 2010). Chapter 4 found no effect of ryegrass intakes above MEm on CH4 yield from cattle estimated by the SF6 technique.

Reports for sheep fed pasture-based ryegrass forages at differing intakes have shown variable and inconsistent relationships with CH4 yield. For example, Molano and Clark

(2008) fed lambs and ewes pasture forages from about 0.8 to 2.0 x MEm, and found no

relationship between CH4 yield and feed intake. In contrast, Muetzel et al. (2009) fed

sheep pasture forages and reported a decrease in CH4 yield of 5.3 g/kg DMI (CH4-

E/GEI of 0.016) for every increase in feed intake above MEm. Sun et al. (2011) fed

sheep ryegrass forages and found a decrease in CH4 yield of 25.6 to 21.5 g/kg DMI

analysis in Chapter 4 found a decrease in CH4-E/GEI of 0.016 with increasing intakes of

ryegrass above MEm from sheep in respiration chambers.

There is some evidence that high intakes of roughage diets may result in a greater increase in rumen volume (Waghorn et al., 1986) compared to concentrates (Mertens, 1987, Waghorn et al., 2002), and roughages will be affected by forage quality. So, the change in MRT for animals fed roughages (or mature forages) may be less than from concentrate diets, or immature forages. Ulyatt (1969) reported that the MRT of the liquid pool for sheep fed immature ryegrass was only 8 h compared to 12 h for mature ryegrass. This, in combination with substrate suitability for propionate production, may partially explain the different responses in CH4 yield to increasing feed intake.

Of the 66 species of methanogens found in a variety of anaerobic habitats, seven have been isolated from the rumen (Janssen and Kirs, 2008). PCR-DGGE allows the compositional diversity of the rumen methanogenic community to be visualised by comparing various band patterns and shifts in methanogenic populations. It has been speculated that rumen methanogen diversity can affect CH4 production (Zhou et al.,

2010), but there has been no direct link between changes in methanogen community diversity and CH4 yield measured from ruminants (Popova et al., 2011). However,

knowledge of the compositional diversity of methanogens in the rumen could enable a more targeted manipulation of the rumen system (Leahy et al., 2010). In this study, no differences in PCR-DGGE bands were observed from sheep fed at low or high feed intakes and further investigation of the link between methanogen numbers and their activity on CH4 emissions is required.

6.4.2 Diet chemical composition

Despite the large range in diet chemical composition achieved by feeding sheep white clover and ryegrass, chemical composition appeared to be of little consequence to CH4

yield, with most the variation predicted by CP concentration (slope of -0.02, R2=0.19). Previous attempts to predict variation in CH4 yield (from unrelated trials) on the basis of

diet chemical composition accounted for up to 51% of variation when pasture forages were fed to sheep, but no relationships could be established for cattle (Waghorn and

CHAPTER 6: Effect of feed intake on methane emissions from sheep 149 Woodward, 2006). A more recent evaluation of CH4 emissions measured in respiration

chambers from sheep fed ryegrass forages in unrelated experiments with varying composition found only 20% of the variation in CH4 yield could be predicted (Chapter

4). Comparisons between white clover and ryegrass forages fed to sheep in three experiments (Chapter 5) showed similar CH4 yields for both diets (Chapter 5), despite

the ryegrass containing 50% more NDF, 80% less pectin, and 40% less CP in the DM, than white clover.

A correlation between CH4 yield and diet composition was also anticipated because diet

composition affects the proportions of VFAs (Bannink and Tamminga, 2005), as well as H2, CO2, and microbial growth. Changes in the products of fermentation alter the

amount of H2 formed, so CH4 formation is likely to vary (Janssen, 2010). Moe and

Tyrrell (1979) and Johnson and Johnson (1995) suggested the fermentation of plant cell walls (i.e. NDF) results in a greater CH4 production compared with non-cell wall

components. Ulyatt et al. (2002b) and Beauchemin et al. (2008) suggested improving diet quality by feeding forages with a lower NDF and higher RFC could also reduce CH4 emissions. However, based on the results presented here, the variation in CH4

yield from sheep was poorly related to the chemical composition of the diet. There is no simple relationship between the diet composition and CH4 yield, but diet

composition affects voluntary feed intake (VFI), and intakes appear to be responsible for an appreciable part of the observed variation in CH4 yields.