Abstract
A meta-regression was conducted on studies published from 2018 to 2023 to explore the effects of nutrient intakes from alfalfa (ALF) and corn silage (CS) on milk yield (MY), energy-corrected milk yield (ECM), N efficiency (NEFF), milk urea nitrogen (MUN), and manure nitrogen excretion (NMANURE) in Holstein lactating cows. The analysis included 193 studies with 698 individual treatment means. Multiple models were developed for each response variable using a backward stepwise approach and cross-evaluated against the database. Nutrient intake from both CS and ALF influenced MY and ECM, with ALF generally having a positive effect. The NEFF was also affected by nutrient intakes from both forages. Generally, greater protein intake reduced NEFF, whereas greater MY was associated with improved NEFF. An increase in the rumen-degradable protein intake (RDPI), especially from ALF, increased MUN. However, no significant effect of protein intake from CS on MUN was observed. Additionally, an increase in crude protein intake and RDPI, regardless of source (CS or ALF), led to an increase in g/d of NMANURE. Our results indicate that nutrient intakes from ALF and CS have different effects on production, excretion, and nitrogen efficiency, supporting their use in targeted nutrient and waste management strategies.