Effects of Dietary Crude Protein Level on Nitrogen Use Efficiency and Urinary Nitrogen Excretion during a Twelve-week period in Late Lactation Dairy Cows

Abstract Text: Our objectives were to determine the impact of decreasing levels of dietary CP and whether there was a treatment by week interaction on intake-N, milk-N (DHI true protein), N use efficiency (NUE, milk-N/intake-N), MUN, and responses related to urinary excretion in late lactation cows. One hundred twenty-eight Holstein cows (means±SD; 736±18 kg BW; 224±54 DIM) were used in a 16-pen study with 8 cows per pen, and fed a TMR once per day (at 8:00 am) for 12 weeks (pen = experimental unit).  Treatments which included diets of 11.8, 13.1, 14.6, or 16.2% CP (DM basis) were randomly allocated to pen for the entirety of the experiment. Rations consisted of approximately 67% forage (half corn silage; half alfalfa silage); soy hulls replaced soybean meal to achieve the desired dietary CP levels for each treatment.  Urine volume was estimated using creatinine concentration in spot urine samples collected 6 hours before feeding for a group of four randomly selected cows in each pen. This protocol was repeated 6 hours after feeding. LS-means of pen-level data presented in the table were obtained on week 2, 8 and 12. Except for urine volume, there was a linear effect for all responses but there were no quadratic effects. There was a treatment by week interaction for most responses. Regardless of treatment, NUE was high and urinary urea-N excretion was low. Under the conditions of this experiment, the 14.6% CP diet allowed for a reduction in urinary urea-N without affecting milk-N.

Keywords: Protein Nutrition, MUN, Urinary Urea

Table. Effect of Dietary CP on measured responses.

	Dietary   CP (DM basis)					P-value1
Item	11.8	13.1	14.6	16.2	SEM	L	Trt*wk
Intake-N, g/d	410d	505c	551b	614a	9.0	<0.01	<0.01
Milk-N, g/d	128c	149b	163.4ab	172.4a	5.04	<0.01	<0.01
Milk-N/Intake-N, %	31.1b	29.4ab	29.6ab	28.0a	0.006	<0.01	0.25
MUN, g/dL	6.3d	8.6c	10.9b	13.47a	0.34	<0.01	0.06
Urine volume, L/d	17.5	18.2	16.8	17.8	0.93	0.95	0.04
Urinary-N, g/L	5.1d	6.4c	7.7b	8.5a	0.24	<0.01	0.74
Urinary-N, g/d	88c	115b	127ab	150a	6.3	<0.01	<0.01
Urinary Urea-N, g/L	2.9d	4.6c	5.8b	6.9a	0.29	<0.01	0.59
Urinary Urea-N, g/d	50d	83c	99b	122a	3.2	<0.01	<0.01
Urinary-N/Intake-N, %	21.6	23.1	23.2	24.4	1.56	0.29	<0.01
a–d  Least squares means within the same row with   different superscripts differ (P<0.05). 1Linear   (L) effect of CP% level in the diet or interaction treatment by week   (Trt*wk).