Fecal phosphorus levels in dry cow, pregnant heifer groups

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Achieving acceptable fecal phosphorus (P) levels for dry cow and heifer groups has been a challenge for some certified nutritionists working with contracted farms through the USDA-NRCS feed management program. At Penn State, we have experienced the same challenges, especially when working with low quality forages and having to supplement with byproduct feeds. Inorganic P is not fed to any of our animals groups with the possible exception of calves receiving calf starter or grower. With this year’s forages, ration adjustments were made to limit the amount of supplemental feed, especially canola meal, for the dry cows and heifers. Fecal samples were taken in July for the lactating, dry cow, and pregnant heifer groups.

The lactating herd for many certified nutritionists is not typically the problem group. Phosphorus fed to the cow’s requirements reflects fecal P on a dry matter basis between 0.55 to 0.80%. In July, the lactating cows were sampled at Penn State. The pens sampled averaged 80 pounds of milk with a 3.70% fat, 3.02% protein consuming 56 pounds of dry matter and 81 grams of P. The fecal P results were 3.65 pounds/ton of P2O5 or 0.50% on a dry matter basis. This is comparable to the average of 3.0 pounds/ton of P2O5 that we have observed for the past nine years. 

For the dry cows sampled in 2011, the fecal P was 7 pounds/ton P2O5 or close to 1.00% on a dry matter basis. Based on the limited research and observations on animals fed to their requirements, an expected range for fecal P on a dry matter basis of 0.65 to 0.85% would be expected. The 2011 results reflect feeding low quality forage and supplementing with additional byproducts to meet the other nutrient requirements for the dry cows. Cows were consuming 61 grams of P with a requirement of 40 to 45 grams. Dry cows were sampled in July 2013 where they were receiving all grass-based forage as either silage or hay with limited amounts of canola meal and a small amount of corn grain to meet energy needs. Cows were consuming 48 grams of P and the fecal P results were 5.26 pounds/ton P2O5 or 0.63% on a dry matter basis.

The pregnant heifers were receiving a ration very similar to the dry cows. They were consuming 32 grams of P. The fecal P results for this group were 5.91 pounds/ton P2O5 or 0.70% on a dry matter basis. The results for both groups illustrate that acceptable fecal P levels are achievable for dry cows and heifers.

Achieving ideal levels of fecal P requires attention to several factors. The whole concept of precision feeding is determining how much these animal groups are consuming. In situations where a total mixed ration is not being fed, this becomes extremely difficult. Also, it appears based on the limited data from the Penn State dairy herd, it does not take a lot of byproduct feeding to elevate fecal P for the dry cows and heifers. The other challenge is how these animals are grouped and how well fecal samples can be taken without contamination from bedding. The good news is ideal levels are possible, but they certainly are not the easiest to achieve.



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