Two areas of concern for dairy producers thinking about nutrient output from their operations are nitrogen (N) and phosphorus (P). These are important nutritional components for optimal production efficiency including lactation, growth and maintenance. Effective strategies for nutritional management of these nutrients exist to reduce their output in manure, while maintaining overall herd productivity.

Nitrogen is the basis for the crude protein level of feeds. Crude protein in the diet supplies amino acids and other forms of nitrogen that contribute to proper rumen fermentation and to support milk production. Excess nitrogen in the diet of dairy cows will be used inefficiently and mostly gets excreted in urine as urea. This is an inefficient use of valuable protein in the diet and contributes to excess nitrogen output in manure.

Dietary protein is expensive and has diminishing benefits for milk production so more is not always better, but reducing levels of dietary protein below recommended levels can lower milk production. A key for nutrient management of nitrogen is that many dairy rations are currently above recommended levels for dietary protein and there are opportunities to reduce protein feeding.

Precision feeding is a strategy being promoted to address this problem in dairy herds. Identifying cow requirements and formulating diets to meet those requirements more accurately is the best current strategy to minimize manure nitrogen output. Researchers recommend analyzing forages and testing dry matter often. High quality forages are important sources of protein and carbohydrates. It is important to balance high quality forage with concentrates that accurately complement them. This approach will minimize excess protein in the diet and result in rations that more closely reflect nutritional requirements.

The accurate grouping of cows for nutritional requirements is another management strategy. Consider a one-group TMR balanced for 35 kg/d of milk production. Many cows in that group will be producing less milk than that and may be over-consuming nitrogen as a result. Generally, only 25-30% of dietary N is converted into milk by the cow, and in situations of excess dietary N, the efficiency is less.

A project at the University of Maryland on the use of milk urea nitrogen testing (MUN) to improve dairy cow nutrition showed benefits for reducing nitrogen excretion. Monthly MUN results and management guidelines that were given to participating farmers resulted in 44% of producers reformulating their rations. Project participants achieved lower MUN values (by 0.52 mg/dl) compared to other farms in the area. The researchers calculated that the 472 farms in the study would save 126 tonnes of excess N per year from entering water resources.

Nutrient management for phosphorus is an easier challenge to meet on dairy farms than nitrogen. Phosphorus from manure can have serious consequences for the environment when not managed properly. The simple solution for phosphorus on dairy farms is to feed less. A recent study indicated that dairy rations in the United States are formulated to contain 0.45 to 0.5% P on a dry matter basis. The latest recommendations (2001) for dairy cattle from NRC are approximately 25% below these values.

Maintaining levels above the new recommendations are done for a variety of reasons. It was convenient to have a safety factor built in to allow for nutrient variation. As well, the old idea that P is necessary at high levels for good reproductive performance needs to be replaced by the new evidence that there is no effect on reproduction by feeding at the lower recommendations. The positive result for reducing typical P levels to required levels is a 30-35% drop in P output in manure. For milk producers, this would mean less money spent on supplemental phosphorus in the diet, continued productivity and a better approach to managing nutrient output.

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