Effects of dietary cowpea (Vigna sinensis) hay and protein level on milk yield, milk composition, N efficiency and profitability of dairy cows

- This study evaluated the effects of substituting 125 g/kg of dietary dry matter from sorghum silage with that from cowpea (Vigna sinensis) hay at both 155 and 170 g/kg of dietary CP in lactating dairy cows under tropical conditions.
- Feeding cowpea hay increased feed and N efficiency and apparent digestibility of dry matter, organic matter, NDF and N, lowered N excretion and increased income over feed cost.
- Reducing CP from 170 to 155 g/kg dry matter did not reduce yield of milk or milk components but reduced feed cost, fecal and urinary N excretion and MUN, and increased N efficiency of milk secretion.
- Decreasing dietary CP and including cowpea hay in the diet of dairy cows reduced both feed costs and N wastage.

Thirty-two lactating Holstein cows were grouped by days-in-milk into 8 blocks of 4 cows and fed 4 diets containing either (dry matter (DM) basis) 0 or 125 g/kg of cowpea hay (Vigna sinensis), added at the expense of sorghum silage, with either low (155 g/kg) or high (170 g/kg) crude protein (CP) supplemented as soybean meal and distillers grains. Diets were balanced to be isoenergetic for cows weighing 540 kg and producing 29 kg/d of milk. Cows were milked and fed ad libitum 3 times a day. The experiment was conducted as a 4 × 4 Latin square design with a 2 × 2 factorial arrangement of treatments; periods were 21 days long with 14 days of adaptation and 7 days of data collection and sampling. Effects of cowpea, CP and their interactions were evaluated using the mixed procedures of SAS, with either pen (traits related to intake) or cow (traits related to production) as the experimental unit. No significant effects of treatment were observed for DM intake (DMI), body weight (BW) gain, and yield of milk and milk components. However, substituting cowpea hay for a portion of the dietary sorghum silage increased milk/DMI and milk N/N intake and decreased milk urea N (MUN) and fecal N excretion, tended to decrease urinary N excretion, reduced feed cost and increased income over feed cost. Moreover, feeding of cowpea hay increased apparent total tract digestibility of DM, organic matter, CP and neutral detergent fiber. Decreasing dietary CP from 170 to 155 g/kg increased Milk N/N intake and decreased MUN, urinary N, fecal N and urinary N/N intake, reduced feed cost and increased income over feed cost. Inclusion of cowpea hay in diets formulated under tropical conditions reduced the need to feed high-cost protein ingredients, improved feed and N efficiency, and reduced risk of N pollution.