Phosphorous for Feedlot Cattle

Livestock Update, January 1999

Mark L Wahlberg, Extension Animal Scientist, 4-H Livestock, Animal and Poultry Sciences, Virginia Tech

Phosphorous (P) is an important mineral associated with cell growth, energy utilization, and maintenance of normal cell fluids. It is also required by rumen microbes for their growth and metabolism. About 80% of the P in the body is found in bones and teeth. The requirement for P for growing-finishing cattle ranges from 0.12% to 0.34% of the total ration dry matter (NRC, 1996). The highest values are for cattle gaining weight in excess of 4 pounds per day.

Phosphorous is a fairly expensive nutrient to add to cattle rations. High P supplements include bone meal, defluorinated rock phosphate, monocalcium phosphate, and dicalcium phosphate. Phosphorous levels in these materials range from 12 to 21%.

Grains are fairly high in P content. In table 1 are shown P values reported by the NRC in the document entitled Nutrient Requirements of Beef Cattle.

Table 1. Phosphorous Content of Feeds Frequently Used In Diets for Feedlot Cattle*

Feed	Percent Phosphorous
Alfalfa Hay and Silage	0.28
Barley Grain	0.35
Corn Grain	0.32
Corn Gluten Feed	0.95
Corn Silage	0.22
Grain Sorghum	0.34
Wheat Grain	0.44

*NRC, 1996.

A good bit of attention has been directed recently towards the P content of feeds, animal waste, and the environment. An enzyme called phytase has been used with swine and poultry to make P in feeds more available to those species. This is because a portion of the P found in plant products is bound to the cell structure in such a way that it is not available for nourishing pigs and chickens. Fortunately, this is not a problem for the ruminants such as cattle or sheep. The microbes normally flourishing in the rumen are able to release this bound P and make it available to the animal. Therefore, the values shown in table 1 can be considered almost totally available to cattle.

This concept was recently proven by researchers in Nebraska. In their study (Erickson et al, 1998a) yearling steers (starting weight 849 lb) were fed rations consisting of 2 levels of Calcium (0.35 and 0.70%). For each Ca level fed there were 5 different P levels evaluated (0.14, 0.19, 0.24, 0.29, and 0.34%). To control P level, ingredients used were corn byproducts that are low in P content. Cattle were implanted with Revalor-S and rations included Rumensin and Tylan. Cattle were fed for approximately 100 days at which they weighed from 1200 to 1250 lb and had .4 inch of backfat.

Feed intake, gain, and feed efficiency were similar across P levels. Average daily gains ranged from 3.87 lb/day for the low P level to 3.38 lb/day for the high P level. Ca level in the ration had no effect on the response from Phosphorous. Neither total bone mineral nor bone strength were affected by P content of the ration .

They concluded that for these yearling steers at this level of gain the P requirement is no more than 0.14% of the ration dry matter.

In a second trial by Erickson et al (1998b), two different groups of cattle were used to evaluate two different rations. One was a control ration consisting of 13.5% crude protein and 0.35% P and the other was a ration balanced using the NRC (1996) system to meet requirements for the specific cattle being fed. The balanced ration used 0.26 down to 0.20% P in the total ration, and crude protein of 11.9 down to 11.5% for yearlings and 12.7 down to 10.8% protein for calves.

Yearlings started at 656 pounds and were on feed 147 days, finishing at 1249 pounds. Calves started at 541 pounds and finished at 1245, thus they were on feed for 193 days. Both groups of cattle were implanted at the beginning and approximately halfway through the feeding period with Revalor-S. They reached approximately 0.5 inch of backfat. The finishing ration contained 7.5% alfalfa hay and the balance was grain, grain by-products, and supplement.

Results are shown in table 2. There was no difference in rate of gain for either group. The balanced ration resulted in lower feed intake and enhanced feed efficiency with the yearlings.

Table 2. Balanced Ration vs Control Ration for Calves and Yearlings.

Item	Yearling Trial - 147 Days on Feed		Calf Trial - 193 Days on Feed
	Control	Balanced	Control	Balanced
Beginning Wt	652	660	539	542
Final Weight	1249	1249	1245	1247
DM Intake	26.2	25.0	20.6	20.5
Avg Daily Gain	4.06	4.01	3.66	3.65
Feed/Gain	6.45	6.21	5.72	5.64

The authors concluded that the NRC (1996) requirement system is effective for determining cattle requirements. Savings in both Protein and Phosphorous resulted by using this system. Cattle performance was not affected by feeding a balanced ration vs feeding P and protein in excess of requirements.

In a companion paper (Erickson et al, 1998c), utilization of nitrogen (N), organic matter (OM), P, and Sulfur (S) were investigated in the trial with the yearling steers shown in table 2. Nutrients were monitored using pen surface core samples, manure volume and analysis, runoff collection, and feed offerings and refusals. All of these samples were analyzed for N, OM, P, and S.

Because rate of gain was the same, then the amount of N, P, and S retained in the animal's body was also the same. However, the amount of intake of these nutrients was less for the Balanced ration, resulting in a lower amount of excretion of these nutrients that was not used for growth by the cattle. Table 3 shows the details.

Table 3. Balance of Selected Nutrients by Cattle fed a Control or Balanced Ration

Item	Nitrogen		Phosphorous		Sulfur
	Control	Balanced	Control	Balanced	Control	Balanced
(Lb/Head)
Intake	74.4	62.7	12.52	7.90	6.46	5.23
Retained	8.1	8.1	2.05	2.03	0.51	0.50
Excreted	66.3	54.6	10.47	5.87	5.96	4.73

The use of a properly balanced ration produced the same amount of beef with much less output of nutrients into the environment and a greater efficiency of use of these nutrients by cattle.

The Bottom Line: Supplementation of Phosphorous to yearling cattle fed a high grain ration is probably not necessary. There is enough P in the grains to meet the requirement of the cattle without supplementation, resulting in a savings to the cattle feeder and a reduction in amount of P in the manure of the cattle so fed. However, those grains are also low in Calcium, so supplementation of Calcium (usually with ground limestone) is necessary to provide adequate levels and also to maintain a proper ratio of Ca:P in the ration. The NRC (1996) model describing nutrient requirements of beef cattle appears to be accurate when dealing with both protein and Phosphorous requirements for feedlot cattle.

References

Erickson, Galen, M Klemesrud, T Milton, T Klopfenstein. 1998a. Phosphorous Requirement of Finishing Yearlings. Nebraska Beef Report, pp 78-80.

Erickson, Galen, T Milton, T Klopfenstein. 1998b. Evaluation of 1996 NRC for Protein and Phosphorous Requirements of Finishing Cattle. Nebraska Beef Report, pp 84-85.

Erickson, Galen, T Klopfenstein, D Walters, G Lesoing. 1998c. Nutrient Balance of Nitrogen, Organic Matter, Phosphorous, and Sulfur in the Feedlot. Nebraska Beef Report, pp 86-87.

National Research Council. 1996. Nutrient Requirements of Beef Cattle, Seventh Revised Edition. Washington, DC: National Academy Press.

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