A team, led by AFBI Hillsborough’s Tianhai Yan, has discovered that the manure nitrogen output associated with 1kg of milk production increased with increasing dietary crude protein concentration and that a dietary protein content of 15% was optimal for milk output whilst reducing manure N output.
“The increase was not significant with diets containing crude protein levels above 150g/kgDM,” said Dr Yan, sharing the results of a large study – to evaluate the effects of dietary crude protein concentration on the efficiency of nitrogen utilisation in lactating dairy cows – with delegates at this year’s British Society of Animal Science’s annual conference.
“Our work indicates that overall dietary crude protein concentration of 150g/kg DM may be appropriate in order to reduce manure nitrogen output, while reducing the decrease in milk yield normally observed with very low protein diets.”
The European Union Nitrates Directives set a limit on the amount (170kg/ha) of manure nitrogen that may be applied to land each year. This limit has significant implications for stocking rates on intensive livestock farms.
“And as a result there is increasing interest in developing mitigation strategies to reduce nitrogen output in faeces and urine in animal production systems,” explained Dr Yan, explaining the rational behind the team’s work –
A large continuous design study, involving 47 first- and 40 multi-lactation Holstein dairy cows, was carried out to examine effects of three dietary crude protein concentrations (180, 150 and 120 g/kg DM) on animal performance and nutrient utilisation from week one to 44 of lactation.
The three mixed diets each contained 450g/kgDM of forage and 550g/kgDM of concentrates. Concentrate supplements consisted of different proportions of the same ingredients – barley, wheat, sugar beet pulp, citrus pulp, soya bean meal, rapeseed meal and molasses.
Measurements of feed intake and faeces and urine outputs were taken during the final six days. And live weight was recorded at the beginning and end of digestibility measurements and milk yield and fat, protein and lactose concentration in milk were measured daily during the digestibility trials.
“And we found that dietary crude protein concentration had no significant effect on live weight or fat, protein or lactose concentration in milk, but increasing dietary crude protein concentration significantly increased DM intake and milk yield,” said Dr Yan.
“Consequently, increasing dietary crude protein concentration significantly increased nitrogen intake, nitrogen outputs in faeces, urine and milk and nitrogen retention.”
The study also revealed that increasing dietary crude protein concentration significantly reduced nitrogen output in faeces and milk as a proportion of nitrogen intake, while significantly increased urine nitrogen output and nitrogen retention as a proportion of nitrogen intake.
