Supplementation of isoacids to lactating dairy cows fed low- or high-forage diets: Effects on performance, digestibility, and milk fatty acid profile.

Research has shown that the dietary supplementation of specific branch-chain volatile fatty acids (BCVFA) (e.g., iso-butyrate, 2-methybutyrate, and iso-valerate), either individually or in combinations with another straight-chain FA, valerate (collectively known as isoacids; ISO), significantly improves cellulolytic bacterial growth in culture media and in the in vivo rumen environment. This acceleration in cellulolytic bacterial growth leads to a surge in cellulase activity in rumen. Moreover, increased cellulase activity facilitates the hydrolysis of glycosidic linkages in cellulose, thereby rendering complex carbohydrates accessible to microbial fermentation, leading to greater fibre digestion. Consequently, nutrient availability for host animals is enhanced, resulting in improved animal performance (e.g., milk production and daily gain). However, the effectiveness of ISO supplementation varies depending on dietary variables such as forage fibre level. Typically, a diet with higher FL enhances the relative abundance of primary cellulolytic bacteria, and ISO supplementation enhances the rumen BCVFA pool, resulting in improved fibre degradability. In contrast, low-forage diets compensate for reduced FL with higher non-forage fibre, which tends to contain more soluble fibre compared with forage fibre and potentially shifts microbial abundance toward more rapidly fermenting bacteria (e.g. amylolytic bacteria at the expense of the slow-growing primary cellulolytic bacteria).

The objectives of the studies cited were to evaluate the response of ISO supplementation on productivity, digestibility, and milk FA profile in mid-lactating dairy cows fed diets with varying levels of forage. Secondly, the objectives were to evaluate the effects of ISO supplementation on feed particle sorting, chewing behaviour, and enteric CH4 emissions in the cows fed low or high-forage diets.

Sixty-four mid-lactating Holstein cows were blocked by parity, days in milk (DIM), and prior milk yield (MY) for multi-lactation cows or genetic merit for first-calf cows, and randomly assigned to one of four diets (n = 16). Diets were arranged as a 2 × 2 factorial, with two fibre levels (FL) containing 21% forage (high forage - HF) and 17% forage NDF (low forage - LF) without iso-acids (WIA) or with iso supplementation (IA; 7.85 mmol/ kg of DM and 3.44 mmol/kg of DM for iso-butyrate and 2-methylbutyrate, respectively). Diets were balanced for similar NEL (1.58 Mcal/kg of DM), CP (16.0%), and total NDF (27.2%). Feed intake and MY were recorded daily. Nutrient digestibility for each cow was determined using indigestible NDF as a marker, and faecal samples were collected at 8-time points (4-h intervals between samples). Enteric CH₄(methane) and chewing activity (rumination and eating time) were

measured using the GreenFeed system and sensor-based ear tag system, respectively. The particle size of each diet and orts from individual cows was measured using the Penn State Particle Separator, and a sorting index was calculated. A sorting index of 100 indicates no sorting, whereas values above or below 100 indicate sorting for or against, respectively. Milk samples from individual cows composited over a 10-wk period were analyzed using the GC for the FA profile.

The ISO did not affect DMI, and LF cows had greater DMI than HF cows (27.8 vs. 26.0 kg/day). However, ISO increased MY (34.7 vs. 37.2 kg/day) and ECM (41.9 vs. 39.0 kg/day) by 7% in cows fed the HF but not in those fed the LF diet, suggesting a FL × ISO interaction. Interestingly, ISO increased ADG (0.4 kg/day) but decreased milk urea nitrogen (MUN) by 9% only in the LF diet as indicated by the FL × ISO interaction. Additionally, ISO increased DM, OM, NDF, and CP digestibility by 10% to 24% in HF, but not in LF (FL × ISO). ISO increased the sorting index for long particles in LF (96.1 vs. 109; P < 0.01) but decreased it in HF (100.8 vs. 92.5; P = 0.04). In contrast, ISO did not affect the physically effective particle sorting index (P = 0.51) or intake (P = 0.27) regardless of FL. In alignment with the long particle sorting index, ISO decreased eating and chewing time in the HF but increased them in the LF diet (P < 0.01). In contrast, rumination time was comparable between FL (P = 0.70) and ISO levels (P = 0.19). In the LF diet, ISO supplementation reduced daily CH4 production (g/day) by 9% and intensity (g/kg of MY) by 18% (P < 0.01). In the HF diet, ISO supplementation led to a 10% increase in daily CH₄(P < 0.01) but did not change CH₄intensity (P = 0.17; g/kg of MY) due to improved milk production. As expected, ISO increased milk odd-chain FA profiles in the IA groups irrespective of FL; for example, the IA had greater C15:0 (1.87 vs. 1.54 g/100g FA) and a tendency for greater C17:0 levels (0.86 vs. 0.76 g/100g FA) compared with the WIA groups.

In summary: ISO improved MY and nutrient digestibility in cows fed the HF diets, whereas it increased ADG and decreased MUN in cows fed the LF diet. ISO also altered feed sorting, feeding behaviour and enteric CH₄ emissions depending on FL. In addition, ISO increased milk odd-chain FA (C15:0 and C17:0) regardless of FL.