Alex Tebbe, Graduate Research Associate, Department of Animal Sciences, The Ohio State University
In a recent meta-analysis conducted at the University of Florida, Husnain and Santos (2019) determined how the crude protein (CP) level fed to dry cows and heifers before calving affects milk production in early lactation. The researchers collected over 125 treatment means from published journal articles. Most studies used Holstein dry cows and heifers. About 20% of those studies provided separate production data for 1st lactation and 2+ lactation cows, which were used to investigate whether effects of protein level differ based on parity. They also collected all diet information and calculated metabolizable protein (MP) intake using the NRC (2001). Husnain and Santos (2019) hypothesized that heifers would require a greater MP intake than dry cows to maximize milk production. This is because heifers are still growing before calving unlike dry cows, and more MP would be required for heifer growth in addition to requirements for maintenance and pregnancy (i.e., fetal and mammary growth).
In heifers, increasing MP intake from 800 to 1,100 g/day linearly increased dry matter (DM) intake after calving (+3.7 lb/day), yields of milk (+2.4 lb/day), milk fat (+0.11 lb/day) and protein (+0.07 lb/day), and body weight (+ 33 lb). The authors found similar responses for MP concentration. A MP intake of 1,100 g/day would be about 11% MP of diet DM or 14 to 15% CP of diet DM for a late gestation heifer consuming 22 lb/day of DM.
In dry cows, increasing MP intake from 800 to 1,100 g/day did not improve intake or milk yields after calving (averages of 43 lb/day of DM intake and 81 lb/day of milk). The only benefit of dry cows consuming greater than 800 g/day of MP was for cows producing >80 lb/day of milk, which had increased milk protein yields (+0.07 lb/day); lower producing cows (<65 lb/day) did not have increased milk protein yield. The authors found similar responses for MP concentration. Around 8% MP of diet DM or 12 to 13% CP of diet DM are adequate for most dry cows.
Overall, results from the meta-analysis confirmed the NRC (2001) recommendations that heifers require more MP than cows. If management conditions allow feeding late gestation heifers and dry cows separately, income over feed costs could be improved by feeding heifers more MP (11% MP of diet DM) compared to dry cows (8% MP of diet DM).
References
Husnain, A., and J. Santos. 2019. Meta-analysis of the effects of prepartum dietary protein on performance of dairy cows. J. Dairy Sci. 102: 9791-9813.
NRC. 2001. Nutrient requirements of dairy cattle, 7th rev. ed. Natl. Acad. Press, Washington, DC.