Critical Evaluation on N balance Studies "Cow-Level" and "Pen-Level"

Instructions: Please answer the following questions assigned to highlight the main points of each article

1) Write down and discuss the N balance equations used in the Spek paper and in the Aguerre paper. Spek paper:
NI = Fecal N + Milk N + Urine N where Urine N was calculated as NI – Fecal N – milk N) for a "zero balance" (as referred to in the paper)
NI = Fecal N + Milk N + Urine N + Unaccounted N (where unaccounted N = “balance” which is often assumed to be N that remained in the body of the gain as lean tissue gain)

Aguerre paper:
NI + N bedding = Milk N + Manure N + Ammonia N

2) Compare the Ammonia N emissions (g/cow per day) reported in Aguerre to the urine Urea N (UUN g/cow per day) reported for cows the NA data set of Spek? What’s the point? UUN in Spek was 168 g/cow per day and Ammonia-N loss in Aguerre was 98-127 g/cow per day. There is a consensus in the literature that most of the UUN is the source of NH3-N emission.

1) Highlight with a few bullet points the main differences in diet chemical composition in EU and NA datasets.
First, NA diets were higher in alfalfa silage lower in NDF and higher in starch;
Second, EU diets were higer in grass silage, higher in NDF and lower in starch;
Third, although EU and NA diets have relatively similar CP, there was 1% higher RUP in NA diets compared with NA diets.

2) Compare the daily excretion of milk N and Urine Urea N (g/cow per day) in the NA and in the EU N flow studies. In both data set cows excrete more UUN than they secrete milk N 3) Using the data presented in Table 1, calculate the Nitrogen Use efficiency of cows in the EU and NA datasets.
EU = 100*133/485 = 27.4%
NA = 100*166/637 = 26.1%

4) Based on the N balance data presented in Table 1, which cow is more “environmentally” friendly?” Explain.
NA cows have the greatest UUN (168 g/d) compared with the EU cow (152 g/d);
NA cows have a lower NUE than the EU cows;
On the flip side, NA cows have lowest UUN per unit of milk N produced compared with EU cows.

5) Among the animal factors investigated, which were the top two most highly associated with UN excretion?
MUN mg/dl and BW (Table 2)

6) Among the dietary factors investigated, which were the top two most highly associated with UN excretion?
CP (% of DM) and RUP (% of DM)

7) When MUN alone was used to predict UN (g/d), what was the predicted increase in UN (g/d) per unit of increase in MUN (mg/dL)?
From Table 3: 11.9 and 14.1 for the EU and NA data, respectively (P < 0.01)

8) When dietary CP alone was used to predict UN (g/d), what was the predicted increase in UN (g/d) per unit of increase in dietary CP (% of diet DM)
From Table 3: 24.8 and 25.5 for the EU and NA data, respectively (NS difference)

9) True or False: The authors found that in the NA dataset there was a significant relationship between CP and DMI, but no such relationship was found in the EU dataset.
True. (See bottom right column page 4316). This explained why adding DMI to the model contributed more to improving model prediction in the EU data compare to the NA dataset.

10) True or False: Cows fed N-free diets do not excrete Urea in their urine.
False. Spek quotes the work of Orskov and McLeod who observed 18 to 26 g/d of UUN when cows were fed N free diet (page 4316 mid left column).

11) Could you speculate as to why DMI contribute to the prediction of UN (albeit moderately) but not to the prediction of UUN?

1) What were some of the assumptions made by Aguerre et al. (2010) when estimating ammonia emission using a mass balance approach for a group of cows?
1) All unaccounted N was lost as NH3-N and no other N-containing compounds (N2O, NO or N2)
2) Change in N in the body of the cow was zero (which was presumed correct on the basis of similar average DIM of cows during each sampling period.)

2) Why did the authors worried about a possible effect of side of the barn?
Because of possible difference in temperature in the north alley and south alley may have created a bias in ammonia emission.

3) The authors indicated that their goal was to maintain the average CP (DM basis) of the recommended (REC) diet between 16% and 17% and to formulate an excess CP diet (EXC) 1.5 units above the REC diet. What were two (very practical) reasons for which the targeted dietary CP ended-up quite different than anticipated throughout the year-long study?
1) Error in mixing concentrate ingredients at the local mill (which was caught later). See section on data and sample collection page 1391.
2) Running out of corn silage in August and September (feed inventory issue) (See Table 1).

4) More wet manure was measured in August and September, the two months during which cows were fed alfalfa silage as the sole forage source. The authors rely on the literature to discuss these results and they argued that this effect was in part due to which of the following? Explain.
a) increased urine volume associated with higher K intake
b) Lower fiber digestibility of alfalfa silage (compared with corn silage)
See end of section on Prediction of manure production page 1397.
Choice a: yes as suggested by Weiss and StPierre data.
Choice b: yes as suggested by data of Table 3.

5) Discuss the implication of the data presented in Figure 4.
Increase N intake did not increase milk N, but increased manure N and ammonia N emission.

6) According to the literature cited, rank the following three housing / manure collection system for their effect on ammonia emission from the lowest to the highest. A) Flushed free stall barn, b) scraped free-stall barn, C) tie-stall barn.
1. Lowest = tie-stall barn.
2. Intermediate = free-stall barn.
3. Highest = water flushed free-stall barn.

7) According to the data of Aguerre, what is the amount of ammonia emitted from the barn floor of a free-stall barn for every g of N going into the milk of the cow when cows were fed the REC and the EXC diets?
0.59 g for the REC diet and 0.82 g for the EXC diet.

8) what are the four main factors influencing ammonia emission in dairy barn according to the literature? (see Monteny et al., 1998).
a) Manure pH,
b) Manure temperature,
c) Manure urea N content,
d) Wind velocity at the surface of exchange (liquid/gas phases).

9) Summarize the overall effects of (a) side of the barn, (b) season, (c) month of the year, (d) manure temperature (e) manure pH and (f) dietary CP as per the results of this study.
No effects of side of the barn, manure temperature, manure pH (not much variation) or season, but high impact of month of the year (depending on forage ingredients fed !) and dietary CP level (REC vs. EXC).

10) True-False: The authors concluded that MUN was a reliable predictor of Ammonia emission.
False. The authors concluded it was not, although there was a linear relationship between NH3 emission and MUN (with an R2 of 0.23 and P<0.08 – see last section of results).

11) True-False: The data provide evidence that MUN was a reliable predictor of not only dietary CP but also manure N excretion.
True. See Figure 2.

Keywords:Critical Evaluation on N balance Studies "Cow-Level" and "Pen-Level"   Doc ID:58740
Owner:Michel W.Group:DS 875 Ruminant Nutrition Physiology
Created:2015-12-06 15:26 CSTUpdated:2015-12-09 15:17 CST
Sites:DS 875 Ruminant Nutrition Physiology
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