Diet-specific conversion methods should be employed in estimating methane emissions from cattle to assist in reducing greenhouse gas emissions, researchers from the University of Manitoba, Washington State University, Colorado State University and the Environmental Protection Agency (EPA) advocate.

Methane is one of the greenhouse gases emitted from livestock and is up to 21 times more potent than carbon dioxide (CO2) in its ability to trap heat in the atmosphere. From manmade sources, agriculture accounts for 50% of all methane production worldwide. Ag's largest methane producer: enteric fermentation — the process of rumination.

Agriculture in the U.S. contributes about 8% of total U.S. greenhouse gas emissions and is the second-largest methane source in the U.S. EPA estimates that, of methane emissions from cattle in the U.S., 58% is derived from the cow-calf sector, 23% from dairy and 19% from feedlot and stocker cattle.

Since measuring methane production in animals requires complex and expensive equipment, prediction equations are widely used to estimate emissions. National estimates of methane emissions rely on mathematical models such as the one recommended by the Intergovernmental Panel for Climate Change (IPCC). However, this method of estimation has been challenged.

Researchers evaluated two empirical and two mechanistic models (COWPOLL and MOLLY) for their prediction ability using individual cattle data.

In dairy cattle, COWPOLL had the least errors and greatest accuracy and precision of predicting methane emissions (correlation coefficient estimate = 0.75).

In feedlot cattle, MOLLY had the least errors, with almost all errors from random sources (correlation coefficient estimate = 0.69). The IPCC model also had good agreement with observed values.

Researchers speculated that a fixed methane conversion factor (Ym) might be an easier alternative to diet-dependent variable Ym. Based on the results, the two mechanistic models were used to simulate methane emissions from representative U.S. diets and were compared with the IPCC model.

The average Ym in dairy cows was 5.63% of gross energy (range 3.78 to 7.43%) compared with 6.5% ± 1% recommended by IPCC.

In feedlot cattle, the average Ym was 3.88% (range 3.36 to 4.56%) compared with 3% ± 1% recommended by IPCC.

Based on these simulations, using IPCC values can result in an overestimate of about 12.5% and underestimate of emissions by about 9.8% for dairy and feedlot cattle, respectively. In addition to providing improved estimates of emissions based on diets, mechanistic models can be used to assess mitigation options such as changing source of carbohydrate or addition of fat to decrease methane, which is not possible with empirical models.

Researchers recommend national inventories use diet-specific Ym values predicted by mechanistic models to estimate methane emissions from cattle.
Kebreab, et al, 2008, Journal of Animal Science, 86:2738.

Heifers fed diets of either whole raw soybeans (SB), wet corn gluten feed (WCGF) or dried distillers grains (DDG) had no differences in artificial insemination (AI) conception rates or final pregnancy rates, say researchers at the University of Nebraska, West Central Research and Extension Center in North Platte.

Their goal was to compare puberty status before synchronization of estrus, response to synchronization and AI and final pregnancy rates in heifers developed on diets containing SB, WCGF or DDG that were formulated to be similar in energy and crude protein (CP). These ingredients vary substantially in fat content, which may affect reproductive performance. Rate of gain during the feeding period and post-AI was also compared.

In a preliminary experiment, 104 crossbred heifers were fed diets containing either 2.75 lbs. of SB/day or 4.4 lbs. of WCGF/day for 110 days (dry-matter basis), beginning at 10 months of age.

In experiment one, 100 crossbred heifers received either 2.75 lbs. of SB/day or 4.4 lbs. of WCGF/day from approximately 7 to 10 months of age (91 days; 4 pens/diet), and then fed 2.75 of SB/day for an additional 114 days (4 pens/diet).

In experiment two, 2.75 SB/day or 2.75 of DDG/day was fed to 100 crossbred heifers for 226 days, beginning at six months of age (4 pens/diet).

At approximately 13 months of age, heifers were fed melengestrol acetate (MGA, 0.5 mg/day) for 14 days, followed by an intramuscular injection of prostaglandin (PGF) 19 days later to synchronize estrus. Heifers (14 months of age) received AI for five days after PGF, at which time the dietary treatments ended. Heifers were commingled while grazing on native pasture and exposed to bulls for about 60 days beginning 10 days after the last day of AI.

Pregnancy to AI was determined by ultrasound 45 days after the last day of AI. Heifers fed SB in the preliminary experiment had lower synchronization rate (81% vs. 96%) and longer interval from PGF to estrus (76.6 vs. 69.2 hours) compared with heifers fed WCGF.

In experiment one, the age at which the heifers were started on SB diets didn't alter synchronization rate (79%) or timing of estrus after PGF (77.8 hours). In experiment two, the synchronization rate (86%) and timing of estrus after PGF (69.3 hours) didn't differ because of diet.

No differences were due to diet for AI conception rates (overall mean for each experiment, 76.5%, 60% and 68.5%), percentage of all heifers becoming pregnant to AI (67%, 46%, 59%) or final pregnancy rates (92%, 90% and 90%) in the preliminary experiment, experiment one or two, respectively.

Therefore, researchers conclude that SB, DDG and WCGF can be used as sources of protein in heifer-development diets at the inclusion rates used in these studies.
Harris, et al, 2008, Journal of Animal Science, 86:476.