Low dietary protein resilience is an indicator of the relative protein efficiency of individual dairy cows.

To try and improve the efficiency of protein use in dairy production has been the goal of many studies in the past 40 years. Lower dietary protein with the same milk protein output should increase protein efficiency and profitability. However, reduced production is a potential risk of feeding diets with less protein, as feeding less protein may reduce the energetic and economic efficiency of producing milk. Thus, the challenge is to identify ways to feed less protein while maintaining or enhancing milk production. One can select for protein efficiency but it is difficult because it requires knowledge of individual cow feed intake, which is difficult to measure on commercial farms. Milk urea nitrogen (MUN) is maybe also an option as an indicator of protein efficiency when cows are fed diets varying in protein sources and content since its measurement requires no knowledge of feed intake. Milk urea nitrogen varies between cows within a herd and has been suggested as a means of selecting for protein efficiency, although there are doubts in the literature. Another challenge for selecting cows based on protein efficiency is that if a cow is fed more protein than she requires, she is essentially forced to use protein less efficiently than her innate ability would allow. In fact, the efficiency of converting metabolizable protein (MP) to milk protein decreases as the MP supply available for milk increases; thus, ranking cows for protein efficiency may have little meaning unless each cow is fed protein exactly at her requirement. One way to examine potential differences among cows for their ability to efficiently use protein is to feed diets that are marginally low in protein. Whether the ranking of individual lactating cows for protein efficiency is repeatable across diets that are high or low in protein content and whether MUN is a meaningful indicator of protein efficiency for cows across diets high and low in protein is however not known. To answer these questions, Drs E. Liu and M. J. VandeHaar designed the experiment described below. The results were published in the Journal of Dairy Science, Volume 103 of 2020, page 11401 to 11412, using the title: Low dietary protein resilience is an indicator of the relative protein efficiency of individual dairy cows. 

A total of 166 lactating Holstein cows in peak lactation with an initial milk yield of 41 ± 9.8 kg per day were fed high-protein (HP) or low-protein (LP) diets in four-week periods in a crossover design with half the cows fed LP first and half fed HP first. The study was repeated with 69 of these cows in late lactation. The LP diets were 14% crude protein (CP) in peak lactation and 13% CP in late lactation and were formulated to contain adequate rumen-degradable protein to maintain rumen function, but inadequate rumen-undegradable protein for cows with average production in this study to maintain their production. The HP diets were 18% CP in peak lactation and 16% CP in late lactation and contained extra expeller soybean meal to meet metabolizable protein requirements. Protein efficiency was defined as the protein captured in milk or in both milk and body tissues per unit of consumed protein. Low dietary protein resilience (LPR) in cows, defined as being able to tolerate lower protein diets and still maintain protein output, was calculated for each cow in peak and late lactation based on actual intake, production and body weight measures.

The results showed that the ability of the cows to maintain total protein captured in milk and body gain when fed less protein varied considerably and the variation was mostly explained by a cow’s captured protein per kilogram of metabolic body weight when fed HP, her lactation number and treatment sequence. Protein efficiency was moderately repeatable across diets within lactation stage. Milk urea nitrogen was not associated with protein efficiency in individual cows within a diet and lactation stage. Cows with greater dietary protein resiliency (higher LPR) had similar protein efficiency on the HP diet as cows with lower LPR, but higher protein efficiency on the LP diet. 

In conclusion, the cows generally maintained their protein efficiency rankings when switching diets between sufficient or insufficient protein; however, some high-producing cows were better able to maintain high production when fed less protein, i.e. having a greater LPR. The authors suggested that LPR might be useful for identifying cows that use protein more efficiently to enhance dairy sustainability. Protein efficiency was poorly associated with MUN; thus, to use MUN to rank cows for protein efficiency within a diet could be misleading.