Nitrogen Management on Dairy Farms
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Nitrogen Losses from a Dairy Herd

In addition to herd size, the nitrogen excreted by a dairy herd is a function of several factors, including:

  1. level of milk production of the herd,
  2. proportion of the total dairy cattle in the herd that are dry cows and heifers, and
  3. amount of excess protein in the rations.

The impact of milk production, herd composition, and ration protein level on N utilization can be seen in table 1. The table shows the effect of each of these variables on N excreted for a 100 lactating cow dairy herd, based on projections with the CNCPS model.

Table 1. Predicted total N produced/day by a 100 lactating cow dairy herd fed a ration with balanced protein and a ration with excess protein.1

  Balanced ration protein2
  Total N Fecal N Urinary N N efficiency3
lbs milk/cow*day
25% cull rate 50% cull rate 25% cull rate 50% cull rate 25% cull rate 50% cull rate 25% cull rate 50% cull rate
47150 51279 29250 31882 17900 19396 22 21
50864 60293 31156 37071 19707 23222 25 23
55663 65092 33213 39128 22449 25964 27 25
  Excess ration protein4
54540 63969 29257 35171 25283 28798 19 18
57676 67106 31163 37077 26514 30029 23 21
60260 69690 33218 39133 27042 30557 25 24

1. On an average day, the herd was assumed to have 100 cows milking, 20 cows dry, and 60 heifers if the cull rate is 25% and 120 heifers if the cull rate is 50%. The cows were assumed to have a mature body weight of 1500 lb. and the milk contained 3.3% protein and 4% fat. Four groups of heifers were maintained (pre-puberty, breeding age, early bred and late bred) and rations were formulated to meet target growth rates to calve at 22-24 months of age and meet a target pre-calving weight of 85% of mature weight. All diets were based on feeds commonly utilized on Northeast dairy farms, including corn silage, alfalfa silage, corn meal, soybean meal, treated soybean meal, whole cottonseed and corn gluten meal.

2. Balanced ration protein: diets for all groups in the herd were computed with the CNCPS model version 5.038 (Fox et al., 2004) in which rumen N and peptides, metabolizable protein, lysine and methionine requirements were met, with only small positive balances.

3. Efficiency is the percentage of intake N for the herd that is synthesized into animal products (milk N + fetal N + growth N).

4. Excess ration protein: 3 x the SD for CP (2.61 percentage units) reported by Chase for the 46 dairy herds on the Excretion page was added as soluble protein to the balanced rations (Chase, 2004). This can occur due to several factors, including excessively high soluble protein in the forage or adding a high CP safety factor to account for large variations in feed composition or requirements of animals in a group.

The following conclusions can be made from the information shown in this table.

In a case study on a dairy farm in New York, Tylutki et al. ( 2004) reported an improved N efficiency from 19% in the base year to 25% for the dairy herd in the 5th year by using precision feeding to address all of these factors in a comprehensive whole farm feed management plan.

Of current concern is the amount of the N excreted that is volatilized in the barn. Factors that affect the percent of N in manure that is volatilized in the barn and lost as ammonia include temperature, ammonia concentration, pH, wind speed and depth of manure on the barn floor. The estimates in table 2 were made based on data collected on a dairy farm in central New York State by Hutson et al. (1998), which account for the primary factors of scrape interval and temperature. This table shows that ammonia volatilization increases the longer the manure lays on the barn floor and the higher the temperature.

Table 2. Estimates of percent of N in dairy manure that is volatilized on the barn floor1.

  Barn temperature, degrees F
Scrape interval, hr
Annual average2 50 40 68
4 1 0 10
8 2 1 19
16 5 2 35
20 10 5 38
22 14 6 39

1. Hutson et al., 1998.

2. Average annual temperature for central New York State.

These percentages applied to the values in table 1 divided by 365 give an estimate of daily volatilized ammonia losses per 100 lactating cow dairy herd with the assumptions used to develop table 1. For example, table 2 gives an estimate for the annual average temperature with an average scrape interval of 12 hr.; these values would be halved or doubled for a scrape interval of 6 or 24 hr., respectively.

Table 3. Estimated daily N volatilization losses (lb/day) on the barn floor per 100 lactating cow dairy herd.1

  Balanced ration protein
  25% cull rate 50% cull rate
lbs milk/cow*day
Total N excreted, lbs N volatilized, lbs Total N excreted, lbs N volatilized, lbs
129 10.3 140 11.2
139 11.1 165 13.2
153 12.2 178 14.3
  Excess ration protein
149 12.0 175 14.0
158 12.6 184 14.7
165 13.2 191 15.3

1. Based on the assumptions and total N excretion values in table 1 and a scrape interval of 12 hr. (8% volatilized) shown in table 2.