Quantifying phosphorus excretion in dairy cows
Dr. Bill Weiss
The Ohio State University

Maintaining nutrient balance for livestock operations requires evaluating the inputs and outputs of phosphorus so that adequate manure storage is available and safe application practices are used.  Analysis of the phosphorus concentration in feed, feed refusals, feces, urine and milk from studies conducted on lactating Holstein cows at the OARDC Wooster Dairy Center were used to develop equations to estimate the output of phosphorus in manure.  To determine manure output, the data from 190 cows which received one of 48 different diets was used.  The phosphorus balance was determined using data from 144 cows which received one of 30 different diets.   These data indicated that fecal and urine output increased with increasing with dry matter intake; however, there was no relationship between manure (fecal and urine) output and body weight.  The data did suggest an increase in urine output as the percentage of corn silage as a percentage of the total forage decreased.  This effect may be the result of higher potassium content of the alfalfa in the forage.  The study concluded that the best way to reduce phosphorus in manure is to feed only enough phosphorus to meet daily requirements.

For specific details, see Bill Weiss’ column in the Buckeye Dairy News online at http://dairy.osu.edu/bdnews/v005iss02.htm.

Discussion: 

Mike Lilburn (OSU) asked if phosphorus (P) availability is the same as digestibility.  Weiss responded that, in theory, the NRC (National Research Council) coefficients of availability are true digestibility, not necessarily availability.  Some of the P available in the diet may still be excreted because it may only pass through bacteria in the rumen. 

Harold Keener (OSU) asked about the accuracy of the equation developed.  Weiss responded that for one specific cow, it is accurate within 10%.  For a group of cows, it is accurate within 2 to 3%. 

Tom Zimmerman (ATI) questioned why the Midwest Plan Service (MWPS) table, which specifies manure excretion per cow, is based on body weight when there is not a good correlation between body weight and manure.  Is the MWPS estimate for manure application of 1 acre/cow needed to maintain P balance accurate?  Weiss noted that the tables are based on the average production for that weight, so the values will vary if there is a difference.  If the average production increases, the number of acres/cow will increase. 

David Munn (ATI) asked whether an increase in hay in the feed may increase urine output since it is dry and scratchy which may increase water intake.  Weiss responded that water intake was not measured in the studies, but as there was little hay in the diet, the increase in urine output correlated to the increase in forage is probably related to the diuretic effect of the potassium in the alfalfa. 

Jay Martin (OSU) asked for a clarification for the total output per cow.  Weiss indicated that the mean output for a lactating cow is 92 lbs/day of fecal matter (wet) or 139 lbs/day of manure (fecal matter (wet) and urine). 

Lynn Willett (OSU) asked Weiss if he felt confident in the data based on the size of the data set.  Weiss responded that he did have confidence in it.  Although there will be some additional statistical analysis, little change is expected.   

Maurice Watson (OSU) asked if milk from Jersey cows has a greater concentration of P than for Holstein’s. Weiss noted that the concentration for Jersey’s is slightly higher. 

Floyd Schanbacher (OSU) asked what type of P was in the manure.  Weiss noted that the type was not analyzed.  It would be a combination of organic, bacterial and indigestible. 

Notes by Mary Wicks 

The OCAMM (Ohio Composting and Manure Management) Program is funded by OARDC/OSU.