Dietary Phosphorus: Economics vs. Environment

Dr. William Weiss, Dairy Extension Specialist, The Ohio State University

Phosphorus (P) is an essential mineral, and when cows consume inadequate amounts of P, feed intake decreases which reduces milk yield.  If the deficiency persists for months, cows are at much greater risk for bone fractures (note: reduced fertility or reproductive efficiency was not mentioned because it is not affected by P deficiency probably until cows start suffering broken legs).  Clearly, we want to feed adequate P to our cows.  Conversely, overfeeding P causes a linear increase in the amount P excreted in manure, and if this manure is not used correctly, manure P can contaminate surface water resulting in substantial environmental damage.  Sources of supplemental P (e.g., dicalcium phosphate or monosodium phosphate) are expensive and can increase diet costs.  The increased feed cost was an added incentive to formulate diets that met but did not greatly exceed the cow’s P requirement (approximately equivalent to diets with 0.35 to 0.40% P on a dry basis). 

The mandated increase in ethanol production has affected several things related to feeding ruminants and that includes P nutrition.  Byproducts such as distillers grains, corn gluten feed, and wheat middlings are often less expensive sources of nutrients than other feed ingredients (see the article on nutrient costs in this newsletter), but these ingredients also have high concentrations of P.  A group from Virginia Tech (Stewart et al., 2012, The Professional Animal Scientist, Volume 28, pages 115-119) formulated numerous diets for a Holstein cow producing 90 lb/day of milk and concluded that high P diets were cheaper than low P diets.  A diet with 0.35% P (usually adequate to meet requirement) costs approximately 4% less per day than a diet with 0.45% P (all other nutrients in both diets were similar and adequate to support 90 lb/day of milk).  With the ingredient prices used in that study, feed costs were about $0.20/cow/day less for a diet with 0.45% P than for one with 0.35% P but only if dietary P increased because of the use of byproducts, not by adding supplemental P.  For the higher P diets, distillers grains and wheat middlings were substituted for a portion of the corn grain and soybean meal.  Feeding a diet with 0.45 vs. 0.35% P is cheaper, but it would increase manure excretion of P by about 25 g/cow/day (about 5.5 lb/day for each 100 cows).  If this extra P in the manure can be used efficiently by crops without contaminating water or causing excessive P build up in the soil, then higher P diets are economically and environmentally sustainable. 

If manure P cannot be used efficiently and correctly, options must be considered to reduce manure P at the farm level.  An obvious option is to feed lower P diets to the lactating cows (but this can increase costs).  Another option is to reduce P excretion by the replacement animals on the farm.  Scientists at the University of Wisconsin (Bjelland et al., 2011, Journal of Dairy Science, Volume 94, pages 6233-6242) conducted a long term study with growing heifers to evaluate the effects of feeding a diet with no supplemental P (approximately 0.30% P) to a diet that contained 0.10% supplemental P (total diet was approximately 0.40% P).  The experiment started when heifers were 4 months of age and ended shortly before they calved.  Absolutely no differences were observed in growth rate (weight, height, girth, and length), reproductive measures (services per conception, age at first breeding, age when pregnant, etc.), health disorders, and first lactation production.  The only thing that differed was that heifers fed high P diets excreted more P in their manure.

Assuming an average DM intake from 4 months of age until 22 month of 20 lb/day, feeding 0.10% unit more P would increase P intake by about 9 g/day and essentially all that would end up in manure.  If a herd has 70 replacement heifers between 4 and 22 months of age per 100 cows, reducing dietary P from 0.40 to 0.30% for those heifers would reduce manure P excretion by about 630 g/day.  This ‘savings’ in manure P excretion from heifers would allow you to increase dietary P in the lactation diet by about 0.03 percentage units (e.g., increasing from 0.40 to about 0.43%) without affecting total manure P excretion from the farm.  A diet with 0.43% P could reduce feed costs compared to a diet with 0.40% P if the extra P came from byproducts.  Environmental and economic sustainability can be achieved but often will require creating thinking.