# Responses to increased dietary contents of non-structural carbohydrates in herbage-based diets for dairy cows: Between- and within-cow ruminal pH variation

## TL;DR

This study shows that dairy cows respond differently to high carbohydrate diets, with some developing lower rumen pH while others maintain stable levels.

## Contribution

The study reveals significant between- and within-cow variability in ruminal pH responses to high non-structural carbohydrate diets.

## Key findings

- Two-thirds of cows experienced ruminal acidification with higher TNC intake.
- One-third of cows maintained or increased ruminal pH despite higher TNC intake.
- Half of the cows showed varying pH responses across repeated trials.

## Abstract

•Between- and within-cow variation in responses to high TNC diets were studied.•Ruminal pH depended on TNC intake, but also on individual cow variation.•Two-thirds of cows showed ruminal acidification with higher TNC intake.•One-third of cows maintained or increased ruminal pH despite higher TNC intake.•Half the cows showed varying ruminal pH responses across repeated trials.

Between- and within-cow variation in responses to high TNC diets were studied.

Ruminal pH depended on TNC intake, but also on individual cow variation.

Two-thirds of cows showed ruminal acidification with higher TNC intake.

One-third of cows maintained or increased ruminal pH despite higher TNC intake.

Half the cows showed varying ruminal pH responses across repeated trials.

While dietary factors, especially high starch intake, are known to promote subacute ruminal acidosis (SARA), individual cows may differ markedly in their susceptibility and response – yet these inter- and intra-animal variations remain poorly understood, limiting targeted management. We investigated the effects of herbage-based diets with moderate (M-TNC) and high (H-TNC) levels of total non-structural carbohydrates (TNC) on ruminal fluid composition, pH, and performance of dairy cows, and their variation between and within animals. Three consecutive crossover experiments were conducted involving 14 animals. In experiments 1 and 3, cows were fed hay and concentrates (experiment 1: 23 (M-TNC) and 35 (H-TNC) % TNC; experiment 3: 18% and 36%). In Experiment 2, cows received either M-TNC (12% TNC) or H-TNC (23% TNC) fresh herbage without concentrate. The overall effects of the H-TNC diet were a greater TNC intake, a shift toward a propionate-enhanced ruminal fermentation profile, and a lower minimum ruminal pH compared to the M-TNC diet. A great between-animal variation in area under the curve at pH <5.8 (AUCpH<5.8) was observed: two-thirds of cows showed increased AUCpH<5.8 with the H-TNC diet, and one-third showed decreased or similar values. Half of the cows included repeatedly in at least two experiments consistently had a greater AUCpH<5.8 with the H-TNC diet than with the M-TNC diet, while the other half showed alternating reactions. Ruminal pH reactions to great dietary TNC contents seemed to vary with diet, individual cow, and time; however, larger studies are needed to confirm these interactions.

Image, graphical abstract