Dairy Herd N Utilization: N Losses

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:

level of milk production of the herd,
proportion of the total dairy cattle in the herd that are dry cows and heifers, and
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
60	47150	51279	29250	31882	17900	19396	22	21
80	50864	60293	31156	37071	19707	23222	25	23
100	55663	65092	33213	39128	22449	25964	27	25
	Excess ration protein4
60	54540	63969	29257	35171	25283	28798	19	18
80	57676	67106	31163	37077	26514	30029	23	21
100	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.

Even with accurate ration balancing, the N efficiency for the herd is lower compared to the 46 herd average shown on the Excretion page for lactating cows (25% for the herd at 80 lb. of milk with 25% replacements/year vs 28.8% for the lactating cows on the Excretion page). This is due to the diluting effect of the low N efficiency for heifers and dry cows (an average of 11%).
Nitrogen excretion increases as milk production increases. However, the efficiency goes up also because more milk N is produced relative to the fixed costs of maintaining the herd.
Nitrogen excretion of the herd increases as more heifers have to be raised as cull rate increases.
Excess protein in the ration adds to the nitrogen excretion, and usually adds to cost of the ration. Managing the feeding program to reduce solubility of the protein in the silages, grouping cattle according to requirements, and reducing variability in the protein content of the feeds fed will allow less protein to be fed.

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 5^th 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
6	4	1	0	10
12	8	2	1	19
24	16	5	2	35
48	20	10	5	38
72	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
60	129	10.3	140	11.2
80	139	11.1	165	13.2
100	153	12.2	178	14.3
	Excess ration protein
60	149	12.0	175	14.0
80	158	12.6	184	14.7
100	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.