A long time ago in university, I came across a faded photograph in our animal science book (Animal Nutrition, Maynard et al, 1979) that showed a poor Venezuelan cow chewing on a bone. Its caption read that she suffered from phosphorus deficiency. I thought at the time that this poor animal was so skinny and looked sick. Little did I know that this picture was my introduction to the importance of phosphorus in grazing beef cows.
Today, most phosphorus deficiencies in beef cows can largely be eliminated by supplementing a commercial loose cattle mineral throughout the entire grazing season. Yet since phosphorus is one of the more expensive ingredients in cattle mineral, producers should choose an appropriate phosphorus-containing mineral that effectively complements the natural phosphorus found in their cow herd’s grazing pastures.
Typical phosphorus requirements of beef cows range from about 18-25 grams per head per day to promote good health, feed digestion, general metabolism and reproduction, and it is usually tied in with a dietary calcium/phosphorus ratio of 2:1. As well, phosphorus requirements are affected by age, the size of the cow, her body condition, pregnancy status and her milk production after calving. For example, high-milk-producing beef cows might need about one to four grams more phosphorus per day compared to the average- or low-milking beef cows.
Subsequent amounts of phosphorus supplied by pasture grass to beef cows are largely based upon the type of forage grazed, time of season and environmental stressors that will reduce phosphorus availability. As grass quality declines throughout the pasture season, so does its phosphorus content. Similarly, drought will also reduce its phosphorus content and thus total intake by beef cows. Timely rains often replenish phosphorus storage in the same forage.
To match the various levels of phosphorus found in grazing pastures, there is a host of commercial cattle minerals formulated with different phosphorus levels from about six to 16 per cent. So when consumed at recommended rates they should help match the beef cows’ essential NRC requirements of phosphorus. However, it should be noted that beef cow requirements from the spring calving stage to the end of post-calving/breeding season dramatically increase, while there is a general decline of phosphorus content in pastures throughout the grazing season.
Case in point
A beef producer releases his cows in mid-May to allow his animals to calve out on lush pasture. Given the cows need about 20 grams of phosphorus per head per day and that his lush pasture only supplies 16 grams of it (a cow eats about 2.3-2.5 per cent of body weight in forage); four grams of phosphorus could be supplied by feeding an eight per cent phosphorus mineral fed at two ounces (56 g), daily. By moving onto the next post-calving stage, the producer might switch to a 12 per cent phosphorus mineral and feed three ounces (85 g) of it to meet significantly higher phosphorus needs and complement lower phosphorus found in mid-summer grass.
It is important that cattle mineral intake is maintained at these recommended rates to assure all requirements for phosphorus are matched. That’s because when a cow eats about three ounces (85 grams) of a 12 per cent P mineral, it should consume about 10 grams of supplemental phosphorus. Therefore, if it is eating only 1.5 ounces (42 grams) of mineral, it is only consuming about five grams of extra phosphorus. The phosphorus gap of five grams could mean the success or failure of getting a fertile cow bred during her post-calving stage.
Keep in mind that beef cows need 15 or so other essential minerals (including vitamins) aside from phosphorus to calve out well, get ready for rebreeding and conceive. Beef cows have parallel requirements for calcium, magnesium, salt and trace minerals — zinc, copper, manganese and selenium. Therefore, a well-balanced mineral including phosphorus should always be provided (two to four ounces per head) to grazing cattle daily.