Effect of partial or total replacement of soybean meal by ground peas in the diet on milk production responses, rumen metabolism and nitrogen excretions of dairy cows

ABSTRACT Soybean (Glycine max (L.) Merr.) meal is the predominant protein supplement in dairy production, but its high cost and considerable C footprint have encouraged the search for alternative protein sources. This study evaluated the effects of replacing soybean meal (S) with ground pea (Pisum sativum L.) grain (P) on productive and metabolic parameters in the diet of dairy cows. Twelve multiparous Holstein Friesian cows (152 ± 18.6 d in milk; milk yield 26.0 ± 1.9 kg d-1) were used; following a 3 × 3 Latin square design, replicated in four balanced squares to account for residual effects. Animals were randomly assigned to one of three dietary treatments: 55% grass silage, 9% alfalfa hay, 17.5% concentrate, 10% ground corn, and 8.5% S (100S-0P); 55.5% grass silage, 9% alfalfa hay, 11.5% concentrate, 4.5% ground corn, 5% S, and 14.5% P (50S-50P); 56% grass silage, 9% alfalfa hay, 7% concentrate, 2.5% ground corn, and 25.5% P (0S-100P). Replacing S with P did not affect DM intake, which averaged 22.3 kg DM d-1. However, milk yield decreased linearly from 23.96 to 22.61 kg d-1 with total replacement of soybean meal (p < 0.05), whereas milk fat concentration increased from 4.99% to 5.17% (p = 0.04). Feed costs decreased from 6.55 to 5.90 USD cow-1 d-1 (p < 0.0001) as the inclusion of peas increased. In the rumen, butyric acid concentration increased from 13.6 to 14.0 mol 100 mol-1, while acetate proportion decreased (p < 0.05). Urinary N excretion tended to decrease from 279.3 to 256.6 g d-1, and N use efficiency showed a quadratic trend, being greatest in cows fed 0S-100P. Partial replacement (50%) of S with P maintained milk production while reducing feed costs and potentially improving the environmental sustainability of dairy systems.