Bone Characteristics of Dairy Cows Fed Diets Containing Different Amounts of Phosphorus

View the project final report

• 2000 annual report

Commodities

• Animal Products: dairy

Practices

• Animal Production: feed rations, manure management, mineral supplements
• Farm Business Management: whole farm planning
• Production Systems: agroecosystems

[Note to online version: The report for this project includes tables that could not be included here. The regional SARE office will mail a hard copy of the entire report at your request. Just contact North Central SARE at (402) 472-7081 or ncrsare@unl.edu.]

Dairy producers typically feed 20% more phosphorus (P) than is necessary to their lactating dairy cows. This results in manure that is high in P, and a form of manure P that is especially vulnerable to runoff into lakes and streams. Bone is an important reservoir for P in the body, and this study confirmed that long term feeding of P at NRC recommended levels (.35-.38% P in diet dry matter) maintained healthy bone structure. This study also demonstrated that feeding P in excess of .37% of diet dry matter did not improve reproductive performance of dairy cows. This research, along with published research, provides a strong justification for removing excess P from dairy diets.

Introduction:

Dairy producers routinely feed phosphorus (P) in excess of the cows' requirement. This is costing the U.S. dairy industry $80-100 million dollars annually, and is resulting in manure that is high in soluble P, a form that is particularly vulnerable to surface runoff into lakes and streams. Since P is the limiting nutrient for algae growth, P is responsible for algae blooms in fresh water lakes and streams. One aspect of understanding the P status of lactating cows is to know when bone is being called upon to supply P. Cows can mobilize up to about 1 kg of bone P when there is demand for P. Normally a cow mobilizes bone P in early lactation, and then restores it later in lactation. It is important that a cow restores her bone P by the end of lactation so she is ready to begin the next lactation.

Many dairy producers feed high levels of dietary P in early lactation to compensate for reduced levels of dry matter intake in early lactation, not recognizing that cows can readily mobilize P from bone to meet a temporary P shortage. Another important reason why dairy producers feed excess supplemental P is the widely held notion that high dietary P will improve reproductive performance of the dairy herd. There is evidence that extremely low dietary P can reduce reproductive performance, but that is likely due to impaired growth of rumen microbes under severe dietary P deficiency. Reduced rumen microbial activity reduces digestibility of the diet, thus diminishing the energy supply. With less microbial activity there is also less protein synthesized by the rumen microbes.

A reduced supply of both energy and protein can interfere with reproductive performance. However, it takes extremely low levels of dietary P before rumen microbes are inhibited, and such low levels are simply not reached with modern dairy diets.

1. To determine if feeding lactating dairy cows a low P diet for 2-3 years affects bone strength and P content.

2. To demonstrate to producers via a field study that lactating dairy cows can be fed 20 25% less P without affecting milk production or reproductive performance. NOTE: This objective was modified after we were more than half way into the project. We started working with five dairy producers, collecting reproductive information on their herds. It was during this time that a popular estrus synchronizing program (Ov-Synch) was being adopted by producers. Three of the five producers that we chose to work with wanted to use the Ov-Synch program. This interfered with our field study objectives, so we decided to conduct an experiment at the U.S. Dairy Forage Research Center research farm which would generate the kind of information needed to persuade dairy producers that they indeed could feed less P without impairing reproductive performance of their dairy herd. Our objective remained the same, but we developed the necessary information differently than we originally planned.