Prediction of ruminal pH from pasture-based diets

Area of Science:

• Ruminant nutrition
• Dairy cattle metabolism
• Forage evaluation

Background:

• Ruminal pH is a critical indicator of digestive health in lactating dairy cows.
• Diets based on fresh pasture present unique challenges for maintaining optimal ruminal pH.
• Understanding factors influencing ruminal pH is essential for maximizing dairy cow performance.

Purpose of the Study:

• To identify predictors of ruminal pH in dairy cows consuming pasture-based diets.
• To examine the relationships between ruminal pH and key animal performance measures.
• To estimate the effective fiber content of pasture for dairy cows.

Main Methods:

• A meta-analysis of 23 studies involving 121 treatment means from lactating dairy cows fed pasture.
• Collated animal and dietary variables to assess relationships with ruminal pH.
• Utilized the Cornell Net Carbohydrate and Protein System (CNCPS) ruminal pH equation to estimate effective fiber.

Main Results:

• Low ruminal pH (<5.8) was associated with increased microbial nitrogen flow, volatile fatty acid concentrations, and milk yield, but decreased milk fat and acetate:propionate ratio.
• Ruminal pH showed positive correlations with forage neutral detergent fiber (NDF) and negative correlations with non-structural carbohydrates.
• No single dietary variable or combination reliably predicted ruminal pH across studies; mean pasture effective fiber was 43% of NDF.

Conclusions:

• Predicting ruminal pH in pasture-fed dairy cows is challenging due to significant inter-study variation.
• Dairy cow performance, including milk and milk fat yield, may not be limited at mean ruminal pH as low as 5.8 on high-quality pasture.
• Effective fiber estimation is crucial for managing ruminal health in pasture-based systems.