Dietary protein is expensive. And as an industry we have generally overfeed protein just a bit so that cows can produce milk to their full genetic potential. Recent research has examined if we can reduce dietary protein without sacrificing milk. But what about all the other ways cows use protein? What about lost body weight from increased tissue mobilization or lost growth in first-calf heifers when the diet lacks needed protein? Can the cow mobilize protein from body stores to overcome a protein deficit long term? And if so at what cost?
At Michigan State University we designed a study to answer those questions. A total of 166 cows were enrolled in the study, 92 primiparous and 74 multiparous. We compared high and low protein diets during peak lactation (50 to 130 days in milk) and during late lactation (190 to 250 DIM). A crossover design was used so that all cows received each treatment. Diets were as follows:
- Peak Milk Low Protein: 14% CP, 31% NDF, 32% starch, 9.8% RDP.
- Peak Milk High Protein: 18% CP, 29% NDF, 30% starch, 9.8% RDP.
- Late Lactation Low Protein: 13% CP, 40% NDF, 26% starch, at least 9% RDP.
- Late Lactation High Protein: 16% CP, 38% NDF, 24% starch, at least 9% RDP.
The high protein diets contained more metabolizable protein than required by the average cow in the study based on NRC (2001). Low protein diets were designed to be deficient and supplied 83 and 95% of the required metabolizable protein in peak and late lactation, respectively. Body weight for all cows was recorded 3 times a week immediately after the evening milking.
During both peak and late lactation cows fed the low protein diets ate less, produced less milk, and gained less body weight and less empty body weight than cows fed the high protein diets. In addition, the low protein diet decreased the digestibilities of DM, NDF and CP by 2.8, 2.8 and 6.2 percentage units respectively during peak milk compared to the high protein diet. The decrease in digestibilities of DM, NDF and CP continued in late lactation with changes of 2.0, 1.8 and 7.2 percentage units.
We also assessed energy losses from feeding the low protein diet. Body weight change from feeding the low protein diet during peak lactation accounted for 43% of the decrease in captured energy and 11% of the decrease in captured protein. In late lactation, body weight change from feeding the low protein diet accounted for 51% of the decrease in captured energy and 14% of the decrease in captured protein. Feeding less protein combined with the reduced DMI did reduce feed cost, but the cost of lost milk and lost body weight was far greater. The loss in net profit, based on current prices, was 27% greater during peak lactation, and 45% greater in late lactation, when losses in body weight and milk were both considered instead of milk only losses.
Feeding less dietary protein impacts more than just milk production. Our data clearly shows that body weight changes can be detected in studies of only 4 weeks. We recommend that body weight change should be routinely measured in studies that evaluate cow response to dietary change in protein content, protein source, or amino acid supplements in order to fully comprehend the total cost effectiveness of the dietary change. In addition, we suggest that nutrition consultants also consider ways to assess the full response of cows to dietary protein changes. To learn more about our research please see the November Journal of Dairy Science.