Low Crude Protein Diets for Lactating Dairy Cows

Crude protein (CP) is a required nutrient for dairy cattle, typically ranging from 15 to 18% on a dry matter basis. Dietary CP is often the most expensive macronutrient, and there’s been considerable interest in reducing the inclusion rate of high protein ingredients to decrease costs. In addition, there’s greater focus on reducing the environmental impact of the dairy industry, with much of the focus on improving nitrogen effi ciency by reducing nitrogen excretion and ammonia emissions. Researchers at Cornell University reported that CP can likely be reduced by 0.5 to 1.5% without negatively impacting milk production while signifi cantly improving income over feed costs and reducing ammonia emissions. Other recently published studies suggest that decreasing dietary CP by approximately 2% will reduce MUN or urinary nitrogen excretion without negatively affecting milk production. However, the diets must be balanced to meet metabolizable protein requirements, particularly the requirements of methionine and lysine in metabolizable protein. Methionine and lysine are co-limiting amino acids in lactating dairy cattle diets when corn and soy ingredients make up the bulk of dietary dry matter.

At Miner Institute we recently conducted a study evaluating the effects of a normal-CP diet containing 16.2% CP compared with a low-CP diet containing 13.7% CP. The diets consisted of 41% corn silage, 10% hay crop silage, and 49% concentrate. The diets also contained 32.2% NDF and 26.7% starch. Dietary CP percent was reduced by replacing canola meal, urea, and some soybean meal with soybean hulls. Both diets were balanced to meet metabolizable protein requirements. However, the normal-CP diet was slightly defi cient in methionine as a percentage of metabolizable protein and the low-CP diet contained the rumenprotected methionine supplement Timet® (VETAGRO, Inc.) to elevate methionine in metabolizable protein. The ingredient changes decreased the cost of the low-CP diet by 40¢ per day. Cows performed well on both treatments. Dry matter intake was not affected by the diets (Table 1).

Low Crude Protein Diets for Lactating Dairy Cows - Image 1

Milk yield was unaffected by dietary treatment, averaging 100.1 lb/day. Milk components were also unaffected by dietary treatment. Milk fat averaged 4.0% and 3.9 lb/day. Milk true protein averaged 3.24% and 3.2 lb/day. Milk urea nitrogen was decreased for cows consuming the low-CP diet (11.3 vs. 13.7 mg/dL). Furthermore, milk nitrogen effi ciency, calculated as nitrogen in milk (lb) / nitrogen intake (lb), was increased for cows consuming the low-CP diet (39.6 vs. 32.6%). Milk urea nitrogen and milk nitrogen effi ciency data suggest that cows consuming the low-CP diet wasted less protein while maintaining a high level of milk production. The minimum amount of dietary CP to achieve high milk production will not be the same for all farms. However, most farms should be able to decrease CP by 1 to 2% units when CP is replaced by fermentable carbohydrates such as high quality highly digestible forage or non-forage fi ber sources. Low-CP diets can maintain high milk production and improve nitrogen effi ciency and income over feed costs as long as the diets supply adequate amounts of methionine and lysine in metabolizable protein.