The Poop on Phosphorus

If we feed too much of this valuable nutrient, it can build up in our soils.

As more farms face the requirement to complete nutrient management plans, on-farm analysis of manure phosphorus (P) and pressures to reduce P build-up in soils will change mineral feeding dramatically. Premixes with no P, fed at one-tenth the current level, are not unrealistic expectations.

Most producers have no immediate need to decrease phosphorus feeding. However, new research is suggesting current levels may be higher than needed, and may harm the environment where excess soil phosphorus contaminates surface and ground water. A high level of phosphorus in rivers and lakes increases the growth of algae. This in turn consumes most of the oxygen in the water and robs other marine life, such as fish, of the needed oxygen to survive.

At last summer's American Dairy Science Association annual meeting, researchers reported on a study in which high-producing dairy cows were fed low levels of dietary P for an extended period. Average milk production, bone strength and composition after three years of low P diets are shown in the table next page. Statistical analysis of the data showed the small differences found among the three groups were not significant.

The highest level, 0.47 per cent, is quite typical of amounts many dairy farms feed today. The middle level, 0.39 per cent, is slightly below National Research Council requirements for high-producing cows. With the level of P commonly found in feedstuffs today, the lowest level of 0.31 per cent can often be achieved in a ration with no mineral supplement.

Nutrition text books teach us that phosphorus is an important factor in energy utilization, with an important role in milk production and reproduction. Its main body reserve is in bones where it plays an important structural role. These are all good reasons to ensure lots of this important mineral in your ration. Yet these researchers have demonstrated that high-producing cows fed less P suffered no ill affects.

Moreover, a recent literature review in Feedstuffs magazine summarizes 13 trials reporting normal conception on diets with 0.30 to 0.39 per cent P.

Phosphorus is the most expensive ingredient in most mineral premixes. Reducing P feeding could save a dairy farmer with 50 cows $1,500 to $2,000 per year. But saving dollars isn't the factor driving research into lower P diets. The amount of P excreted in manure prompts all the interest in this area.

Nutrient management plans for manure have become the keystone in defining environmental responsibility for livestock farmers. The concept central to these plans is that manure application rates should balance the nutrients in the manure applied with the uptake by the crop grown. So, if the amount of a nutrient going onto an acre of land in the form of manure, or fertilizer, balances with what comes off in the form of crops, there'll be no build-up of that particular nutrient. If crops use up all nutrients, very little will become polluted run-off.

In nearly all cases the most difficult element to balance, and the one that defines the acreage needed, is phosphorus.

We can apply the concept of phosphorus balance to cows and to the whole farm, as well as to the acres of cropland. What goes in one end must come out the other in one form or another.

For example, let's take a cow consuming 22 kilograms of a diet with 0.45 per cent phosphorus and producing 40 litres of milk per day.

Here's what goes in:

Here's what goes out:

Another two or three grams may eventually leave as part of a cull cow. The milk and manure account for nearly all the P.

At the whole farm level, there must be a balance as well.

Here's how P arrives at the farm:

Here's how it leaves the farm:

Some of the numbers are included here because calculating a whole-farm phosphorus balance can be an interesting exercise. If the amount arriving on your farm exceeds the amount leaving, P is building up in your soil.

Someday, manure nutrient management will become a requirement for all livestock farms. When that day comes, building up P in your soil will end. Without doing the math, if you produce all your own feed, sell no crops, purchase no commercial fertilizer and feed enough mineral premix to replace what goes out in the milk truck, you would be in balance. When all feeds are home grown, the balance point would occur when you bring in 150 grams per cow per day of a 16 per cent phosphorus premix. If you don't grow your own soybeans, and buy one kg per cow per day of soymeal, your balance point would be 100 grams of the premix.

In reality, most dairy farms import far more phosphorus than they export. As a result, P levels have been building up in soils for many years. Feed analysis results that show increasing levels of P in forages confirm this trend. While to this point higher soil levels may have improved crop yields, many farms are now reaching the high to excessive levels implicated in water pollution.

Many dairy farms purchase little or no fertilizer. The main source of the excess P coming on to these farms today is feed, especially in the form of mineral premix or the mineral component of commercial supplements.

For decades the prevailing nutrition philosophy has been that if a little is good, a lot is good insurance. In future this may be insurance we can't afford.