Effect of concentrate on nitrogen turnover and excretion of P, K, Na, Ca and Mg in lactating cows rotationally grazed at high altitude

The effects of concentrate on N and mineral excretion were determined in 2 X 12 early-lactating cows rotationally grazed at 2000 m above sea level for 77 days in two seasons A and B. Supplementation was oriented towards maintenance energy requirements to account for the extra requirements on alps. Two concentrates equally high in easily-fermentable energy (8.7 MJ NEL) but differing by 94 g/kg DM in absorbable protein ( maize gluten) were fed at three respective maintenance energy:protein levels as 1.0:0.8 (A/ B), 0.5:0.4 (A) and 1.0: 1.8 (B). A control group received no concentrate (A/B). Herbage and N intake was reduced in cows supplemented with approximately 4 kg of the low protein concentrate (1.0:0.8). Milk N excretion increased improving N utilisation by 52%. Excretion of urine N and its proportion of manure N decreased by 44 and 18% with this type of concentrate. Supplementing extra protein (1.0:1.8) reduced effects on N utilisation and excretion relative to controls, although herbage intake was similarly reduced as with the low-protein concentrate. Concentrate reduced the excretion of faecal Ca and Mg, and urine K. Seasonal N and P fertilisation rates from excreta were low. Supplementary feeding increased fertilisation rates per hectare of P and K whereas N only increased with additional protein. (C) 2001 Elsevier Science BY. All rights reserved.