21 May 2013

5 minute read

Phosphorus in Livestock Diets - Economics and Environment

Not so many years ago, we provided or advocated supplemental phosphorus (P) to most livestock just to make sure we had enough. By 'we', I include government feed regulators, feed industry, consulting nutritionists, producers and academia. Since then, we have learned that excess P has environmental and economic consequences.

This is because environmental P is implicated in algal blooms in surface water and is one of the primary reasons P from agriculture is addressed under Ontario's Nutrient Management Act. Only about one-third of ration phosphorus leaves the farm in the form of meat or other livestock products, meaning that the other two-thirds accumulate on the farm.

This is a good thing, as this means the majority of P becomes or remains part of the cropping/forage cycle and will eventually boost the P test of all feeds - cash crops and forages - grown on the farm.

Grains Are High in Phosphorus

Corn, barley, wheat and other feed grains routinely tests around 0.3% P. By comparison, the P requirement for an 800 lb. steer gaining about 3.75 lbs/day is about 0.25%. As a percent of ration, P requirement trends down as cattle become bigger, and is reduced at lower rates of gain.

An important development on the P front is the advent of distiller's grains and other grain by-products like corn gluten feed and steep water. By the very nature of the fermentation process, the P (and many other nutrients) is amplified three-fold in distillers' grains. Therefore, rations that use distillers are certainly another case were P is not limiting. In fact, in many cases there is excess P. The grain-based cattle finishing industry has learned that it can comfortably formulate rations without supplemental phosphorus.

Ontario Forages May Have Enough Phosphorus

So what lessons can be learned by the forage based sectors like cow-calf and backgrounders from the grain feeding sector? Perhaps that it is time to re-evaluate the practice of routine P supplementation here as well, in particular, because it is an expensive nutrient to supplement. In the past, much prominence was given to the idea that a high P vitamin-mineral premix aids fertility in beef cows. It would appear those recommendations originated in range conditions and not intensive farming conditions as seen in Ontario. The requirements for a lactating beef cow and a backgrounding (pasture) steer are about 0.22% and 0.20% respectively and have been addressed in previous editions of VB (Volume No. 8 Issue No. 23, July 2009).

Included are four leads or pieces of information that demonstrate many Ontario pasture and forages will, in fact, have enough P for beef cows and grass cattle without P supplementation. The only way to know is to sample your forages and get them analyzed:

1. 2012 Forage Summary from A&L Labs, London
2. Forage Summary from SGS Labs, Guelph
3. Historic (2002) Feed Summary, OMAFRA Factsheet
4. 2012 Grey-Bruce Pasture Data as in Figure 2.

Figure 1. Many Ontario pastures and conserved forages fed to beef cows are high enough in phosphorus (P) to enable mineral programs that include zero supplemental P as has been implemented with high grain rations over the last decade.

What these four data sources indicate is that there is a significant body of evidence that shows forages routinely test above 0.25% P, and many test towards and over 0.3% P, a trend that was also evident in previous years as evident in the OMAFRA link in number 3. Most of these average values exceed the minimum threshold of 0.22% needed, indicating many of the individual lab analyses also will. There is a chance that data sources 1 to 3 (laboratory results) are biased towards dairy operations due to their increased likelihood of participating in forage testing. Source number 4, however, represents beef grass farms only.

Text Equivalent of Diagram

Phosphorous data taken on Grey and Bruce County pastures in 2012 as part of an OCA funded study on pasture trace mineral levels is shown in Figure 2. The graph depicts the requirement for lactating cows of 0.22% P as a horizontal red line. The nine right-most column represent pure species from the same paddock of one farm. Two properties clearly indicate they are low in P and require supplementation, two are periodically low, and the remainder met the requirements over time and likely require no supplemental P for lactating cows.

Trace Minerals Are Needed

Keeping in mind that P is only one nutrient in a multitude of individual essential mineral elements in a 'mineral premix', it remains important that the mineral supplementation addresses elements that are indeed deficient or limiting. Ontario is almost universally deficient in selenium, so that is one that needs to be done right. Salt, zinc, copper and other trace-minerals may also be limiting. In cases of confinement or winter feeding, vitamins A, D and E also likely warrant attention. So, this is to say that just because one mineral element can be removed from the mix, it does not mean that a 'mineral' supplement can be neglected due to these other nutrients needing attention. Then of course there are also the very clear cases where P is limiting and must be supplemented as in Figure 2.

So… Do or Don't Your Cows Need P?

Supplemental P for livestock (as monocalcium phosphate or dicalcium phosphate) is a significant expense. There is a good chance you don't need it! The only way to tell is to undertake forage testing and build a library of tests over time such that you can begin making informed long-term mineral supplementation decisions on your home-grown pastures and forages. Using this information you and your feed advisor or feed industry partners can save some money on individual mineral constituents you don't need, and focus your efforts on minerals or feed additives you do! 'Test and Supplement' as required is the word on phosphorus in your cow-calf nutrition program.

Further Reading You can view the full report by clicking here.

April 2013