Yes, indeed, the moisture (or dry matter content) affects cow intake. Researchers and practitioners alike have determined that preparing a ration too dry (or too wet) reduces appetite. If you do not have much silage in the ration or your feeds are on the dry side (a typical situation), adding water to achieve about 50 to 55% dry matter would be preferred.

Here's a formula and an example for determining how much water (gallons) to add per ton of total mixed ration (TMR) mix:

For example: To take 70% DM (30% water) to 55% DM (45% water) would require about 65 gallons of additional water.

Step 1: Take actual dry matter as a percent/desired dry matter as a percent
Example: 70 (actual dry matter)/55 (desired dry matter) = 1.27

Step 2: Multiply answer from Step 1 by 2000
Example: 1.27 x 2000 = 2540

Step 3: Answer from Step 2 minus 2000
Example: 2540 - 2000 = 540 lb of additional water per ton of TMR mix

Step 4: Answer from Step 3 divided by 8.35
Example: 540 lb water/8.35 = 64.7 or 65 gallons of water/ton TMR mix