Environmental benefits of feeding maize grain

Reduced dietary nitrogen

Throughout the majority of the year New Zealand’s ryegrass -clover pasture contains more than 20% crude protein. At times, especially in the spring and during the autumn flush, pasture protein level can exceed 30% crude protein. Lactating cows require 14 – 18 % crude protein to support milk production. While nitrogen excretion in milk and dung increases linearly with dietary nitrogen intake, nitrogen concentration in the urine increases exponentially as nitrogen intake increases9, 10. Put simply, the more nitrogen cows eat above requirements, the more they excrete in the urine and urinary nitrogen levels may be as high as 1000 kg N per ha.

Figure 6. The relationship between daily total N intake and N output in dung, milk and urine10

Environment Waikato data shows that for a farm producing 850 kgMS/ha, using 100 kg fertiliser N with effluent applied to the land, 69% of the nitrogen loss comes from urinary nitrogen. Feeding a low protein feedstuff such as maize silage (7.5% crude protein) or maize grain (8% crude protein) in conjunction with high protein pasture dilutes dietary protein content and reduces nitrogen excretion by the cow (Table 3). Maize grain can be grown in lower risk areas and fed in sensitive catchments to decrease dietary and therefore urinary nitrogen.

Table 3. Effect of feed source on N output in milk, dung and urine in absolute and relative terms (in parenthesis)11
Type of silage  N intake* N output (kgN/cow) (% intake)
  (kgN/cow) Milk Dung Urine
Lucerne 37 6 (16) 8 (22) 23 (62)
Pasture 24 6 (25) 7 (29) 11 (46)
Cereal 16 6 (38) 5 (31) 5 (31)
Maize 12 6 (50) 3 (25) 3 (25)

*Based on 1tDM/cow.

9 Castillo et al, 2001. The effect of protein supplementation on nitrogen utilisation in lactating dairy cows fed grass silage diets. Journal of Animal Science 79 :247-253.

10 Kebreab et al, 2001. Nitrogen pollution by dairy cows and its mitigation by dietary manipulation. Nutrient Cycling in Agroecosytems 60:275 - 285.

11 Ledgard, S. 2006. Nitrogen management - why is it important and what can we do about it? Proc. 2006 Dairy3 Conference 4: 23-31.