The variation in VFA production among parities has not been fully studied or explained, however, Bedford et al., (2020) have shown that feed intake affects VFA production with particular reference to propionate, where reduced feed intake can lead to reduced production. Furthermore, Rahman et al. (2013) reported that the molar proportion of different fatty acid production depends on the structural composition of the feed ingredients. Variations in VFAs could also be associated with total organic carbons (TOCs). Feeds with lower TOCs would produce lower VFAs (Rahman et al., 2013). From the current study, young animals produced more acetate, propionate and butyrate compared to older animals (P3 and P4). The effects of parity on VFAs is not fully understood, and is a subject needing further study.
The differences among seasons on VFA production could be related to differences in ambient temperature. Similar conclusions have been hypothesised by Bedford et al. (2020) in which they said high ambient temperature relates to low feed intake and heat stress, thus shifting rumen VFA absorption and transport dynamics across the rumen epithelium. With regards to our study acetate, propionate and butyrate are proportionately higher in wet and cold seasons, while isobutyrate, isovaleric and valeric acid are higher in the dry season. This is suggestive of the fact that both temperature and rainfall have significant effects on VFA production. The interactive effects between parity and season on VFA production was not expected, however this also demonstrate the need to gather more information on rumen fermentation characteristics prior to methane evaluation. Seasonal variations in VFA concentration in the current study are an indication of the effect of diet composition, exceptionally high values close to 200 mM have been reported when animals graze on fresh grass or when fed starch- rich diets (Mwangi et al., 2022). The increase in molar concentrations of isobutyrate, isovaleric and valeric acid from wet-cold to dry season was interesting. This has been promoted by Mwangi et al. (2022), when they reported that increased total rumen VFA positively correlates with increased diet digestibility. Following this line of thinking during the cold and dry seasons, grasses have matured and the proportion of ADF is significantly higher hence digestibility becomes retarded, since ADF is negatively correlated with digestibility (Mwangi et al., 2022; Washaya et al., 2021, 2018).
The effect of parity on methane production in beef is poorly understood, however, it is known that heavier animals have higher maintenance requirements hence tend to eat more and therefore, produce more CH4 (Negussie et al., 2017). Our results have shown that there is an interaction between parity and season with regards to methane yield (MY). Animals in P2 generally produced lower MY irrespective of season while those in other parities had higher values in wet and cold seasons compared to the dry season. Interestingly animals in P3 had the highest MY values, this is in contradiction to the fact that heavier animals would produce more CH4 (Demarchi et al., 2016; Negussie et al., 2017; Washaya et al., 2018; Orcasberro, 2021). Hence a controlled study to evaluate and prove this fact is recommended for future studies. Furthermore, the body condition of the animal significantly influence their enteric methane production, with fat animals producing less CH4 than lean animals irrespective of their body weight. This is so because body fat metabolism utilizes less energy than protein metabolism hence, fat animals require less feed intake for maintenance (Negussie et al., 2017).
It is hypothesized that CH4 emissions are positively associated with the acetate: propionate ratio (Negussie et al., 2017), and result from the current study also confirms this assertion. When the ratio was high, less methane was produced and this was also significant among parities and seasons. The stoichiometric relationships between VFA production and production of H2 is crucial in ruminant animal production. It is believed that higher acetate and butyrate values are associated with increased methane production and fibrous diets, while propionate production is directly linked with lowered methane yield and grain based diets ( Orcasberro, 2021; Bittante et al., 2017; Manriquez et al., 2019; Samir & Mourad, 2015; Soul et al., 2019; Souza et al., 2020). This is so because propionate is generally the main electron sink within the rumen, however, Rosa et al. (2021) promoted that other electro-accepting pathways would increase with decreasing CH4 yield, resulting in the formation of butyrate, valerate, caproate, ethanol and propanol. These dynamics are imminently important for ruminant animals as they address the issue of acidosis (Prathap et al., 2021; Souza et al., 2020)
Using the first two equations, the MY falls below the recommended average of 23.3g/kgDM (Intergovernmental Panel on Climate Change IPCC, 2019) for grazing animals. However, the MY values for the third equation were higher than this recommendation, hence we recommend that it might be the best method of determining MY for grazing animals. Nonetheless, higher MY values are indicative of poor forage quality. Orcasberro, (2021) suggested that higher methane production was associated with feeding C4 species, that have a higher content of non-structural carbohydrates and lignin, which lower intake and slows the passage rates. The variation in MY over seasons has also been reported (Demarchi et al., 2016; Orcasberro, 2021). This is linked to type of grass and the extent of lignification, C4 grasses are generally tougher compared to C3, while lignification tend to promote resistance to physical digestion of grass by rumen microbes.