Estimating nitrogen excretion and deposition by lactating cows in grazed dairy systems

Authors: AARONS, SHARON - Agriculture Research And Development, Department Of Economic Development, Jobs, Transport And Resou; GOURLEY, CAMERON - Agriculture Research And Development, Department Of Economic Development, Jobs, Transport And Resou; Powell, Joseph; HANNAH, MURRAY - Agriculture Research And Development, Department Of Economic Development, Jobs, Transport And Resou

Submitted to: Soil Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/5/2017
Publication Date: 8/21/2017
Citation: Aarons, S.R., Gourley, C.J., Powell, J.M., Hannah, M.C. 2017. Estimating nitrogen excretion and deposition by lactating cows in grazed dairy systems. Soil Research. 55:489-499.

Interpretive Summary: Increasing animal densities on grazing-based dairy farms has increased reliance on imported feeds, which has increased the amount of manure nutrients that need to be effectively recycled on the farm. Quantifying animal nutrient intakes and manure nutrient excretion is difficult due to challenges in estimating pasture dry matter intake by grazing cattle. This paper reports on research undertaken to quantify manure nitrogen excreted by lactating herds, the relative importance of excreta nitrogen compared with other nitrogen inputs to dairy farms, and manure nitrogen loads deposited to different locations around grazing-based dairy farms in Australia. By interviewing farmers from 43 different diverse grazing system farms on at least 6 occasions, we found that: 1) these herds received more than half of their energy requirements from supplementary feeds despite the grazing base of the dairy systems; 2) dietary nitrogen intakes were well in excess of those recommended in the literature, resulting in excess nitrogen in the manure; and 3) nitrogen use efficiency in these herds was lower than what can be achieved with different management systems. Results demonstrated that quantifying manure nitrogen loads and spatial nitrogen distribution by lactating cows can assist in improving nitrogen management in grazing system dairy farms.

Technical Abstract: Large nitrogen (N) surpluses are a feature of most grazing system dairy farms in Australia, as observed elsewhere globally. An increase in animal densities and greater reliance on purchased feeds also mean that nutrient inputs in feeds have increased; therefore, the role of grazing animals in N flows and deposited N loads is larger. However, quantifying animal N intake and N excretion is difficult due to challenges in estimating pasture dry matter intake by grazing cattle. This paper reports on research undertaken to quantify manure N excreted by lactating herds, the relative importance of excreta N compared with N inputs to dairy farms, and N loads deposited to different locations around grazing system farms. We collected data by interviewing farmers on at least six different occasions from 43 diverse grazing system farms located in temperate, Mediterranean, sub-tropical and tropical environments. We modified an animal performance method for estimating annual dry matter intake to calculate daily N intake and excretion and animal feed N use (NUE, milk N/N intake). The results indicated that these herds received a mean of 52% of their energy requirements from supplementary feeds despite the grazing base of the dairy systems. Mean N intakes (545 g/cow/day) were well in excess of dietary intakes recommended in the literature, resulting in excretion on average of 433 g N /cow/day in these systems, and low NUE (21%, 11-39%; mean, minimum-maximum). The feed N intake, excreted N, and NUE data were used to develop regression relationships which were generally similar to the prediction equations reported in the literature for confinement-based dairy systems. Calculated annual N flows through the lactating herds were 69% of total N inputs onto these farms. When applied to the specific locations visited by the lactating herds, deposition of excreted N to pasture paddocks was not uniform. Almost 50% more N was returned to paddocks that were on average closer to the dairyshed, and about twice as much N was returned to feedpads and holding areas as that deposited in dairysheds and yards. On average, 20% more N was deposited on paddocks by the lactating herd (168 kg N/ha) than was applied as fertilizer (141 kg N/ha). As manure was not collected from half of the holding areas and feedpads, considerable N accumulation and potential N loss is expected within 150 m of dairysheds and yards on these grazing system farms. These results demonstrated that quantifying excreta N loads and spatial nutrient distribution by lactating cows can assist in improving N management in grazing system dairy farms.