Consumption of a daily ration formulated to contain the current Na requirement set by the NRC appears to provide too much Na for lactating cows under tropical conditions. A tentative value of 1.2 g kg(-1) DM is proposed as the Na requirement for dairy cows under tropical conditions.

Table of Contents Chapter 1 General introduction Chapter 2 An initial investigation into the relationship between ration composition and milk production of dairy cows in Thailand: A cross sectional study Chapter 3 An attempt to define the sodium requirement of lactating dairy cows in a tropical environment Chapter 4 Response of salivary Na/K ratio to changing Na supply of lactating cows under tropical conditions Chapter 5 Effect of dietary protein levels on rumen metabolism and milk yield in mid-lactating cows under hot and humid conditions

A daily Na intake of heat-stressed lactating cows to a ration intake of 1.6 g Na kg DM was insufficient to restore Na deficiency. One week was sufficient to deplete heat-stressed lactating cows of Na, allowing for rapid dose-response studies utilizing the salivary Na/K ratio as a parameter for Na status of cows under tropical conditions. © 2016 Society of Chemical Industry.