Effects of feeding tropical forage legumes on nutrients digestibility, nitrogen partitioning and performance of crossbred milking cows

Eight crossbred (Brown Swiss × Creole) lactating cows were used in a replicated 4 × 4 Latin square design to test the effects of four diets on milk production, apparent total tract digestibility (aTTd), nitrogen balance and profitability. The diets differed in the main forage and nitrogen sources: 1) sorghum silage as main forage and soybean meal as main nitrogen source (control); 2) mixed sorghum + jackbean silage with reduced proportion of soybean meal in the diet (jackbean-silage diet); 3) sorghum silage and cowpea hay with reduced proportion of soybean meal in the diet (cowpea-hay diet); 4) sorghum silage as main forage and urea substituting a proportion of soybean meal in the diet (urea diet). The experiment lasted 84 days, with four periods including 14 days of adaptation and 7 days of data collection. Individual milk yield and dry matter intake were recorded. Total feces and urine excretions were estimated from spot samples, and samples of milk, feed, urine, and feces were collected and analyzed. Diets were designed to fulfil the requirements of cows producing 11 kg milk/d, but average production was 9.6 kg/d, therefore cows in this experiment were likely overfed on energy and protein. Dry matter intake was higher for the cowpea-hay diet compared with all other diets, but milk yield and composition was similar across all treatments. There were no differences in aTTd of dry matter and neutral detergent fiber between the diets. Crude protein aTTd was lowest for the jackbean-silage diet and highest for the urea diet; whereas aTTd of organic matter was lowest for the jackbean-silage diet and highest for the cowpea-hay diet. Milk urea-nitrogen concentration was highest with the control diet and lowest with the jackbean-silage diet. Similarly, nitrogen use efficiency (g milk nitrogen/100 g nitrogen intake) was highest when feeding jackbean silage. No effects of diet were observed for nitrogen excretion in urine, but nitrogen excretion in feces was highest when feeding both legumes. Both diets containing forage legumes had lower feed costs and resulted in a tendency for a higher benefit-cost ratio. Even though more research is needed to better understand the characteristics of forage legumes as sources of protein, and their effects on animal performance and nitrogen excretions, these results show that jackbean silage and cowpea hay at inclusion levels between 250 and 300 g/kg dry matter have potential to substitute soybean meal in diets of crossbred cows producing 10 kg milk daily