Dr. Bill Weiss, Dairy Nutrition Specialist, Ohio State University
The amount of forage eaten by a lactating dairy cow is a function of dry matter (DM) intake and the concentration of forage in the diet. The DM intake can be accurately estimated for a herd, but the concentration of forage in the diet depends on economics, nutrient content of forages and other ingredients, and production and health goals of individual farms. The values presented below are for an 'average' farm and should be adjusted for specific situations.
Table 1. Expected dry matter (DM) intake and annual forage DM needed (tons) for lactating cows at different milk yields (lb/day per lactating cow).
|
% of Dietary DM that is a Specific Forage
|
|||||
| Milk Yield |
DM Intake
|
10%
|
20%
|
30%
|
40%
|
| Holstein cows |
(lb/day)
|
(tons of forage DM needed per 365 days per lactating cow1)
|
|||
| 60 |
47
|
0.9
|
1.9
|
2.8
|
3.8
|
| 80 |
54
|
1.1
|
2.2
|
3.3
|
4.3
|
| 100 |
61
|
1.2
|
2.4
|
3.7
|
4.9
|
| Jersey cows |
|
|
|
|
|
| 40 |
35
|
0.7
|
1.4
|
2.1
|
2.8
|
| 50 |
39
|
0.8
|
1.6
|
2.3
|
3.1
|
| 60 |
43
|
0.9
|
1.7
|
2.6
|
3.5
|
1Assumes 10% shrink during storage and feeding and to account for feed refusals. In some situations, shrink could be as much as 20%.
To calculate the amount of a specific forage needed by the lactating herd, the forage numbers in the table should be multiplied by the average number of lactating cows in the herd. For example, if a Holstein herd averages 100 cows that are being milked, average production is 70 lb/day (average for only lactating cows) and the diet contains 40% corn silage (DM basis), 100 cows x 4.05 (average of 3.8 and 4.3 in the table) = 405 tons of corn silage DM is needed to feed the lactating cows for the year. Divide that number by the proportion of DM in the silage to obtain the amount of as-fed silage needed. For example, if the silage averages 35% DM, 405 tons/0.35 = 1157 tons of as-fed corn silage will be needed for the lactating cows.
If the forages fed to lactating cows are also fed to dry cows and heifers, those amounts need to be added. Expected DM intake for a dry Holstein cow is 30 lb/day and 22 lb/day for a dry Jersey cow. The amount of lactating cow forage needed by dry cows = DM intake x the expected concentration of lactating cow forage in the dry cow diet x the average number of days cows are dry x average number of dry cows. For example, if the dry cow diet is expected to be 25% corn silage (same silage as fed to lactating cows), the dry period averages 60 days, and on average you have 15 dry cows, the corn silage needed by dry cows (Holstein herd) would be 30 x 0.25 x 60 x 15 = 6750 lb (3.4 tons). That value should be adjusted for shrink (10%) and added to the number obtained for the lactating cows. On average, DM intake will be 2.2 to 2.3% of body weight for a growing heifer that is gaining at an appropriate rate. Therefore, on average (assuming a normal distribution of ages), a herd of growing Holstein heifers will consume about 16 lb/day of DM (11 lb/day for a Jersey heifer herd). Multiplying the appropriate number by the concentration of lactating cow forage in the diet and by the number of heifers and 365 will equal the amount of lactating cow forage fed to growing heifers. For example, a Holstein herd averages 100 heifers and the diet is 20% corn silage (same silage as fed to lactating cows), the corn silage needed for heifers will be 16 x 0.20 x 100 x 365 = 58.4 tons of DM. After adjusting for shrink (10%), the amount needed is 64.2 tons/year.
In summary, the amount of a specific forage needed for lactating cows is calculated using numbers in Table 1. The amount (if any) of that forage fed to dry cows and growing heifers is calculated and added to the lactating cow number. The sum equals the amount of a specific forage needed by the farm for a year. The actual forage needed by specific farms could be as much as 10% higher.