Cattle Protein & Reproduction

While all aspects of nutrition impact reproductive performance in cows, protein (nitrogen) metabolism plays a unique role in these interactions. There is a wealth of research data outlining the problems caused by inadequate dietary protein, as well as a growing pool of information relating to the effects of excess protein on fertility.

Protein Deficiencies

Many of the forages fed to beef cows are unable to supply adequate protein to the cow, or to provide enough rumen-degradable protein to meet the needs of the fiber-digesting microbes. Dietary protein deficiencies lead to poor body condition, and cows that either do not breed back, or breed back later than desired. These cows are liable to have weaker calves, and produce limited, less effective colostrum. They may also produce less milk through lactation. This results in lighter calves weaned, both in the current and subsequent calf crops.

Additionally, a shortage of DIP (ruminally Degradable Intake Protein) limits microbial activity. And as rumen fermentation is decreased, so is total dry matter intake and digestion. So, with less forage consumed, and breakdown of that forage less complete, the supply of all nutrients is depressed.

What about an Excess?

In order to stimulate and maintain high levels of milk production in early lactation, many dairies balance rations at or above 20% crude protein. Recent research, primarily with highproducing Holsteins, has shown a link between excess dietary protein and some measures of reproductive efficiency. In a trial at Louisiana State University, 23% protein diets fed after calving resulted in reductions in first service breeding and overall pregnancy rates. Similarly, work at Cornell showed that when high dietary protein levels were reflected in elevated plasma urea nitrogen (PUN) levels, pregnancy rates were decreased.

However, in another Cornell experiment, researchers found that they were not able to significantly elevate PUN levels, even when feeding 150% of the degradable protein requirement, unless they also restricted energy intake to 70% of the cows' requirement. At that point, they were able to correlate high PUN with reduced first-service conception and a potentially harmful drop in uterine pH. But it appears from this work that properly matching energy and protein supplies may help offset the negative impacts of excess protein.

Because high levels of rumen-degradable protein or NPN would be expected to have a more direct impact on plasma urea levels than by-pass protein, researchers at Oregon State University compared the effects of 21% protein diets with different proportions of DIP. But they concluded that the excess dietary protein in their experimental diets had the same impact on reproductive performance, regardless of degradability.

Why Is Excess Protein a Problem?

First, it is important to remember that all this work has been done with cattle genetics, diets, and a level of milk production that bear minimal resemblance to a typical beef cow operation. But dietary protein fed well in excess of requirements for maintenance, pregnancy, or milk production does seem to have the ability to:

Ø       Have a direct effect (e.g., lowered pH) on the uterine environment;

Ø       Alter hormone secretion (possibly impacting insulin, progesterone, and/or prostoglandin);

Ø       Lead to a protein:energy imbalance, which would be worsened as some of the already limited energy would have to be wasted in the process of converting ammonia to urea prior to recycling and excretion.

Are There Particular Concerns With NPN (Urea)?

The short answer is no. A five-year, 85,000 cow summary based on Michigan DHIA records showed that dairy cows receiving urea in their diets exhibited the same calving interval and % open cows as those that did not. And a review of research done by Dr. J.T. Huber of Michigan State showed that neither services per conception, nor first-service conception rate, were impacted by feeding urea. In fact, if we look all the way back to the 1950's, researchers showed that cows that received their entire dietary crude protein supply in the form of urea were able to reproduce normally.

Nevertheless, the belief that urea per se can harm reproductive performance is still encountered in the industry. Perhaps this is because a direct link between dietary urea and blood urea seems logical. In fact, all DIP sources-- both 'natural' proteins and non-protein nitrogen -- yield ammonia in the rumen. All the molecules in this ammonia pool are equally available for uptake by the rumen microbes, or movement into the bloodstream if this doesn't happen. Blood ammonia is converted to urea in the liver, and may be recycled or excreted. PUN levels rise when there is more ammonia released in the rumen than the microbial population can effectively utilize, regardless of source. We see this with excess total crude protein intake, pulse dosing of rapidly degraded crude protein sources, and/or inadequate energy (carbohydrate) available to the microbial population.

Management Solutions

For the cow/calf producer, this is an issue that resolves itself with a well-balanced nutrition program. Forages and supplements need to be selected that:

Ø       Supply adequate protein for maintenance, gestation, and lactation . . . in a cost-effective manner

Ø       Don't grossly overfeed protein in the diet . . . if your forages are of that high a quality, dilute them with a low-cost alternative

Ø       Supply adequate energy in a form that enhances activity of the fiber-digesting bacteria ideally in the form of sugar and/or soluble fiber, with minimal starch

Ø       Provide a relatively steady supply of nutrients to the rumen environment . . . don't pulse dose urea or other highly soluble protein sources.

Source: Dr. Cathy Bandyk, Quality Liquid Feeds