Dietary Chemical Components and Enteric Methane Production

Dietary Chemical Components and Enteric Methane Production: Comparison

Please note this is a comparison between Version 1 by Michel Galyean and Version 2 by Lindsay Dong.

Methanogenesis is critical in cattle because it prevents accumulation of metabolic hydrogen in the rumen by serving as a reducing equivalent sink. Alternative hydrogen sinks exist, however, and these alternative sinks are affected by the ingredient and chemical composition of the diet, such that the quantity of CH₄ produced by cattle varies based on dietary constituents that are fed. Diets that produce acetate liberate hydrogen to be used by methanogenic archaea to produce CH₄. Conversely, propionate serves as a net hydrogen sink, and diets that increase propionate and decrease acetate result in decreased ruminal CH₄ production, reflecting decreased availability of metabolic hydrogen for methanogens to reduce CO₂ to CH₄.

cattle
diet formulation
dietary chemical components

1. Introduction

Beef cattle production is the single largest agricultural commodity area in the United States, contributing over USD 66 billion in receipts in 2019 [1]. Although cattle can convert low-quality feeds into high-quality protein for human consumption, they are a source of agricultural greenhouse gas emissions to the atmosphere. The agriculture sector in the United States contributes approximately 10% of total greenhouse gas emissions, and livestock contributes 3.8% [2]. Nonetheless, enteric CH₄ emissions are responsible for 30% of the anthropogenic methane budget, highlighting the need for a clear understanding of factors that affect CH₄ production and development of practical mitigation strategies.

Methanogenesis is critical in cattle because it prevents accumulation of metabolic hydrogen in the rumen by serving as a reducing equivalent sink [3]. Alternative hydrogen sinks exist, however, and these alternative sinks are affected by the ingredient and chemical composition of the diet, such that the quantity of CH₄ produced by cattle varies based on dietary constituents that are fed. Diets that produce acetate liberate hydrogen to be used by methanogenic archaea to produce CH₄. Conversely, propionate serves as a net hydrogen sink, and diets that increase propionate and decrease acetate result in decreased ruminal CH₄ production, reflecting decreased availability of metabolic hydrogen for methanogens to reduce CO₂ to CH₄.

2. Dietary Chemical Components and Enteric Methane Production

2.1. Relationships of Methane Production to Dry Matter Intake and Dietary Chemical Components

Results of the mixed model regression analyses are shown in Table 1, with graphical representations of the relationships shown in Figure 1. Because of the importance of DMI as a driver of enteric CH₄ production, initial analyses involved regression of daily CH₄ production on DMI. As expected, the relationship between these two variables was strong, with DMI accounting for 82.1% of the variation in daily CH₄ production (Table 1; Figure 1). Dry matter intake has consistently been identified as a key component of equations to predict CH₄ production in cattle ^[4][5][6][7][9,10,11,12], with DMI alone often yielding prediction equations that are equivalent in accuracy and precision to more complex equations.

Figure 1. Relationships between study-adjusted enteric methane production (g/d or g/kg of dry matter intake) and dry matter intake DMI; (A), dietary crude protein (B), ether extract (C), neutral detergent fiber (D), starch (E), starch:neutral detergent fiber ratio (F), and diet metabolizability (metabolizable energy/gross energy; (G) developed from a literature database.

Table 1. Relationships between study-adjusted enteric methane production (g/d) and dry matter intake (DMI) and methane production expressed as g/kg of DMI and various dietary chemical components and diet metabolizability developed from a literature database ¹.

	Regression Coefficients		Regression Statistics
Item ²	Intercept	Slope	RMSE	r²
	---- CH₄, g/d ----
Dry matter intake, kg/d	26.0477	15.3710	13.96	0.821
p-values ³	<0.001	<0.001	CV = 11.39%
Lower 95% CI	20.2892	14.4950
Upper 95% CI	31.8062	16.2470
	---- CH₄, g/kg of DMI ----
Crude protein, %	20.2005	−0.0344	2.53	0.003
p-values ³	<0.001	0.381	CV = 12.82%
Lower 95% CI	19.0317	−0.1115
Upper 95% CI	21.3694	0.0428
Ether extract, %	22.2295	−0.5871	2.31	0.150
p-values ³	<0.001	<0.001	CV = 11.57
Lower 95% CI	21.5201	−0.7577
Upper 95% CI	22.9390	−0.4165
Neutral detergent fiber, %	13.5959	0.2001	2.02	0.696
p-values ³	<0.001	<0.001	CV = 9.65
Lower 95% CI	12.9563	0.1840
Upper 95% CI	14.2355	0.2162
Starch, %	23.4214	−0.1060	2.04	0.495
p-values ³	<0.001	<0.001	CV = 9.89
Lower 95% CI	22.9950	−0.1191
Upper 95% CI	23.8478	−0.0929
Starch:neutral detergent fiber ratio	22.7962	−2.4587	2.18	0.662
p-values ³	<0.001	<0.001	CV = 10.91
Lower 95% CI	22.4363	−2.6730
Upper 95% CI	23.1561	−2.2444
Metabolizability	34.8909	−23.6630	1.84	0.561
p-values ³	<0.001	<0.001	CV = 8.80
Lower 95% CI	33.3687	−26.2140
Upper 95% CI	36.4131	−21.1120

¹ Data were adjusted for random intercepts and slopes associated with the 63 studies in the database. ² Dietary chemical composition data were expressed on a dry matter basis. Metabolizability = metabolizable energy divided by gross energy. ³ Probability that the intercept and slopes differ from zero; CV = RMSE divided by the overall mean of dry matter intake, dietary chemical components, and metabolizability, expressed as a percent; r² is not adjusted for the number of parameters in the model.

2.2. Managing Methane through Feeding Intake Management Strategies

Energy requirements of ruminant livestock are sufficiently well defined to allow “programming” of DMI to meet the needs for a given level of production. Because of the importance of DMI as a driver of enteric CH₄ production, the use of restricted feeding to limit over consumption or programmed feeding to achieve a particular rate of body weight gain for feedlot beef cattle might offer a potential avenue for the feedlot industry to decrease CH₄ production ^[8][23]. In addition to decreasing enteric CH₄, feeding management approaches that decrease DMI would be expected to decrease fecal output, thereby also decreasing CH₄ losses via manure ^[8][23]. Moreover, effects of decreasing DMI through feeding management should be additive with other CH₄ mitigation approaches such as feed additives that inhibit methanogenesis. Nonetheless, for feedlot cattle, decreasing DMI also carries a risk of extending the days on feed to reach a particular carcass weight and composition endpoint or negatively affecting meat quality indices like marbling. Such increases in the length of the feeding period or decreased product quality could affect the economics of production and potentially negate decreases in enteric and manure CH₄ production associated with management of feed intake, requiring careful evaluation of this approach as a CH₄ mitigation strategy. An alternative to managing DMI as a mitigation strategy would be the selection of more efficient animals (e.g., cattle with low residual feed intake) ^[9][24]. Opportunities to modify CH₄ through management of DMI in lactating dairy cattle are likely more limited than for feedlot cattle. Feeding higher concentrate diets could decrease CH₄ production in lactating dairy cows, both through altered ruminal fermentation shifting metabolic hydrogen away from CH₄ and through decreased DMI needed to maintain desired production levels. Nonetheless, decreasing the level of NDF from roughage to allow for lower DMI would likely have a negative effect on milk quality, specifically milk fat content and would possibly have negative effects on animal health through an increased risk of acidosis, rumenitis, and systemic inflammation ^[9][10][11][24,25,26]. Thus, although management of DMI to lower CH₄ production offers possibilities in lactating dairy cows, it would need to be combined with other dietary formulation and feed additive strategies for successful application in practice.

2.3. Applications to Diet Formulation for Mitigation of Methane Emissions

Based on the results of the regression analyses described previously, the key dietary factors to consider in formulation strategies to decrease enteric CH₄ production would be concentrations of NDF, starch, and diet metabolizability, with dietary EE concentration being of lesser importance and dietary CP concentration having virtually no effect. Practically, these key factors are often interrelated in terms of diet formulation. Mixed diets with decreased NDF concentration often have increased starch concentration (i.e., a decreased forage and increased grain), which also results in an increased ME concentration and thereby increased metabolizability. For confined cattle fed mixed diets, changing the forage:concentrate ratio is widely recognized as a feasible CH₄ mitigation strategy ^[9][24]. With all-forage diets for example, a variety of factors affect NDF concentration, including forage type and maturity. More digestible forages decreased CH₄ yield in dairy cattle and sheep, but effects were less clear for beef cattle ^[12][27]. Nonetheless, increased forage quality generally decreases CH₄ production per unit of animal product because DMI and animal production typically increase as forage quality increases ^[9][24]. Increased digestibility of higher-quality forages also would be expected to decrease manure CH₄ losses. Type of forage can be important, as greater CH₄ yield was reported for C4 vs. C3 grasses and warm-season legumes ^[13][28]. As noted previously, feeding diets with a greater concentration of starch is a repeatable approach to decrease CH₄ yield and should also decrease CH₄ associated with manure. Starch generally decreases enteric CH₄ because methanogens are sensitive to low ruminal pH ^[14][29] and feeding starch results in a lower ruminal pH than feeding all-forage diets [15]. Even so, Beauchemin et al. ^[9][24] observed that the global capacity to increase grain feeding to ruminants is limited, so using increased dietary starch as a mitigation tool is limited to production systems in which grains are normally fed at high levels. Grain type (e.g., horny vs. floury endosperm) also can affect starch digestion ^[16][30], with lesser starch digestion with a greater proportion of horny endosperm, although steam flaking can offset the negative effects of endosperm type ^[16][30]. Heat and moisture processing methods like steam flaking increase gelatinization of starch and increase the ruminal proportion of propionate and decrease ruminal pH, thereby decreasing CH₄ yield ^[16][17][30,31]. Although adding dietary fat sources has been extensively studied as a tool for decreasing CH₄ yield ^[9][24], and the regression analyses showed a negative relationship between dietary EE concentration and CH₄ yield, the relationship was highly variable and of low predictive value. With potential negative effects of fat on fiber digestion noted previously, as well as relatively high cost of fat sources, careful consideration should be given to the total concentration of fat in the diet, as well as to the sources of fat added to the diet. It should be noted that for practical implementation of any dietary formulation approach to mitigate CH₄ yield, feed mixing and delivery, as well as potential sorting of feed by animals are issues of concern. If diets are inadequately mixed, thereby resulting in the consumption of feed with variable concentrations of particular nutrients, benefits of dietary mitigation strategies would be decreased. Similarly, diets or feeding practices that promote sorting of feed ingredients by groups of cattle could negate the effects of dietary management strategies.