Mineral Programs for Beef Cows Grazing Pasture

April 2008

After salt and supplementation of magnesium oxide in grass tetany areas, mineral supplementation programs can vary. Soils mineral profiles impact mineral profile in the forage and therefore impact the minerals that need supplemented. Also, forage maturity impact mineral composition. The 1996 NRC for Beef Cattle suggests there are at least 17 minerals required by beef cattle. There are minerals that interact with one-another and therefore impact their utilization and these need to be factored into a supplementation strategy.

Phosphorus is a required mineral by beef cattle and is one of the most expensive minerals in a mineral supplement. So how much phosphorus supplementation is needed for cows grazing pasture this summer?

Phosphorus needs for beef cows is impacted by milk production and forage type. High-milk females require more compared for average- to low-milk females. The 1996 NRC indicates that the phosphorus needs average about 22 grams per day for the first 6 months post-calving, with a range from 25 grams per day to 18 grams per day for a female who's peak milk production is 18 lb per day and she weighs 1200 lb. If she consumes 2.5% of her body weight dry matter basis during June grazing and 2.3% in August and 2.1% in October and summer range is .17%, .16%, and .15% phosphorus in June, August, and October, how much P is needed from the mineral supplement. In June, she's getting 23 gram/day form the forage, so if she eat 2 oz. of a 6% P mineral supplement it would supply another 3.4 gram (2 oz/16 oz per lb x 454 gr/lb x .06 P in the mineral = 3.4 gr) of P which meets her P requirement. In August let's assume that the average dam is 5 months post-calving and she needs 19 to 20 gr/day P. Calculations based on intake and P content of the forage indicates she is getting 20 gram/day from the forage (1200 lb cow x .023 x .0016 x 454 grams/lb = 20 gr per da). In October, milk production has declined substantially, and warm-season range quality is also declining and the cow needs 17 to 18 grams of P per day. If she is consuming 2.1% of per body weight and the forage is .15% P, she is getting 17 gram per day form the forage.

Minerals are important for beef cows. Over-feeding them is not economical. Phosphorus is an expensive component of a mineral program. In my example, supplementation of P through the breeding season for cows grazing warm-season pasture in Nebraska seems needed and after that it is hard to pencil in. Contact your state beef specialist to see if they have mineral profiles of common grasses in pastures in your state and then do the calculations.

Dr. Rick Rasby, Professor of Animal Science
Animal Science, University of Nebraska–Lincoln, Lincoln, NE