Effects of diet protein concentration and forage source on energy and nitrogen balance

REYNOLDS C.K.; CROMPTON L.A.; MILLS J.A.N.; HUMPHRIES D.J.; KIRTON P.; RELLING A.E.; MISSELBROOK T.H.; CHADWICK D.R.; GIVENS D.I.

Parma

Simposio; 3rd EAAP International Symposium on Energy and Protein Metabolism; 2010

Increasing concerns regarding nitrogen and methane emissions from dairy cattle are bringing pressures on producers to develop feeding strategies for reduced nitrogen and methane excretion relative to milk yield.  The objective of the present study was to determine the effect of forage source (maize or grass silage) and dietary protein concentration on energy and nitrogen balance in lactating dairy cows.   Six Holstein-Friesian dairy cows in mid-lactation were fed ad libitum total mixed rations consisting of a 50:50 mixture (dry matter [DM] basis) of forage:concentrate, with the forage comprised of either 25:75 or 75:25 grass:maize silage.  Concentrates were formulated to provide diet CP levels of approximately 140, 160 and 180 g per kg ration DM, in a 2 x 3 factorial experiment.  Cows were randomly assigned to diets in a 6 x 5 incomplete Latin Square design with 4 week periods.  Measurements of respiratory exchange and energy and nitrogen balance were obtained in the last week of each period.  Measurements of N2O and ammonia emissions from respiration chambers were made to account for possible losses of N during balance measurements, although urine was acidified to prevent N loss.   Feed DMI was increased by feeding the high-maize diets and by increasing dietary protein, but the effect of protein differed with forage type.  Digestion of feed DM was increased by increasing dietary protein and was greater for the high-grass diet, thus faecal DM excretion was reduced.  In contrast, urine volume was increased linearly by dietary protein.  Milk energy yield was greater for the high-maize diets.  Milk protein yield were increased by feeding the high-maize diet, whilst milk fat yield increased with increasing dietary protein.  There was no effect of dietary protein on milk protein yield, but urine N excretion increased linearly with increasing dietary protein and was greater for the high-grass diets.  Faecal N excretion was increased by dietary protein for maize-based diets, but not grass-based diets.   Methane excretion was not affected by diet, but methane excretion per kg DMI and per kg milk yield was greater when the high-grass diet was fed.  There was no measureable emission of N2O from the chambers, but ammonia emitted in exhaust air increased with increasing diet protein concentration and ammonia trapped in air conditioner condensate was greater for maize than grass-based diets.  Statistical analysis and interpretation of the results continues