Strategic grazing management decreases nitrogen excretion intensity of dairy cows

Batalha, Camila Delveaux Araujo; Congio, Guilhermo Francklin de Souza; Santos, Flávio Augusto Portela; Silva, Sila Carneiro da

doi:10.1590/1678-992X-2020-0251

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Note • Sci. agric. (Piracicaba, Braz.) 79 (2) • 2022 • https://doi.org/10.1590/1678-992X-2020-0251 copy

Strategic grazing management decreases nitrogen excretion intensity of dairy cows

Authorship SCIMAGO INSTITUTIONS RANKINGS

ABSTRACT:

There is limited information regarding both nitrogen (N) and energy partitioning of dairy cows grazing well–managed tropical pastures. The objective of this study was to investigate the N and energy partitioning of mid–lactation dairy cows on rotationally grazed elephant grass using two pre–grazing targets: 95 % or maximum canopy light interception (LI_95% or LI_Max) during regrowth. The study used 26 Holstein × Jersey dairy cows arranged in a randomized complete block design with three 40–day periods of sampling. Grazing at LI_95% increased organic matter and crude protein intake by 20 % (p ≤ 0.05) which resulted in a 9 % increase in fat corrected milk yield (p ≤ 0.05) relative to LI_Max. Cows grazing at LI_95% had greater concentration of total volatile fatty acids, butyrate and valerate (p ≤ 0.05), and smaller acetate (p ≤ 0.05) than those grazing at LI_Max. Intake of net energy for lactation (NE_L) and NE_L secreted in milk were greater (p ≤ 0.05), while partitioning of NE_L towards maintenance tended to be greater (p = 0.07) for cows grazing at LI_95% than those grazing at LI_Max. Milk urea nitrogen and both urine and fecal N excretion were greater for cows grazing at LI_95% (p ≤ 0.05), but N excretion intensity was lower than in cows grazing at LI_Max (p ≤ 0.05). Strategic grazing management using the LI_95% pre–grazing target increases N losses through both urine and feces; however, it reduces N excretion intensity of dairy cows by 9 %.

Keywords:
canopy light interception; elephant grass; energy partitioning; nitrogen partitioning; pasture–based systems

Item	Treatments		SEM	p-value
Item	LI_95%	LI_Max	SEM	Trt	Per	Trt × Per
OM	89.6	88.8	0.54	0.28	0.02	0.68
CP¹ 1 Data from Congio et al. (2018).	21.0	19.4	0.50	< 0.001	< 0.001	0.04
A + B1² 2 protein fractions adapted from Sniffen et al. (1992): A + B1 = non-protein nitrogen + rapidly degraded protein, B2 = protein with intermediate rates of degradation, B3 = slowly degraded protein and C = unavailable N; EE = ether extract; NFC = non–fibrous carbohydrate calculated as [100 − (NDF + ash + CP + EE)]. , % of CP	24.3	23.1	1.38	0.55	< 0.001	< 0.01
B2² 2 protein fractions adapted from Sniffen et al. (1992): A + B1 = non-protein nitrogen + rapidly degraded protein, B2 = protein with intermediate rates of degradation, B3 = slowly degraded protein and C = unavailable N; EE = ether extract; NFC = non–fibrous carbohydrate calculated as [100 − (NDF + ash + CP + EE)]. , % of CP	39.8	39.7	1.61	0.94	< 0.001	< 0.001
B3² 2 protein fractions adapted from Sniffen et al. (1992): A + B1 = non-protein nitrogen + rapidly degraded protein, B2 = protein with intermediate rates of degradation, B3 = slowly degraded protein and C = unavailable N; EE = ether extract; NFC = non–fibrous carbohydrate calculated as [100 − (NDF + ash + CP + EE)]. , % of CP	31.0	32.1	1.03	0.46	0.08	0.73
C² 2 protein fractions adapted from Sniffen et al. (1992): A + B1 = non-protein nitrogen + rapidly degraded protein, B2 = protein with intermediate rates of degradation, B3 = slowly degraded protein and C = unavailable N; EE = ether extract; NFC = non–fibrous carbohydrate calculated as [100 − (NDF + ash + CP + EE)]. , % of CP	5.4	6.0	0.59	0.54	0.04	0.30
EE	3.1	3.2	0.19	0.68	< 0.01	0.12
NFC	3.9	3.2	0.80	0.34	0.76	0.77

Item		Treatments		SEM	p-value
Item		LI_95%	LI_Max	SEM	Trt	Per	Trt × Per
Intake, kg d^–1
	Total DM¹ 1 Data from Congio et al. (2018).	18.2	15.9	0.61	< 0.01	0.11	0.22
	Forage DM¹ 1 Data from Congio et al. (2018).	12.3	10.1	0.52	< 0.01	0.01	0.18
	OM	16.2	13.5	0.55	< 0.001	0.68	0.11
	CP	3.3	2.7	0.12	< 0.001	0.01	0.88
	NDF	8.6	7.6	0.35	0.04	0.03	0.11
Digestibility, %
	DM	62.9	57.8	0.64	< 0.001	0.23	0.27
	OM	67.1	60.6	0.81	< 0.001	0.10	0.12
	CP	69.9	65.2	0.86	< 0.001	< 0.001	0.11
	NDF	56.4	50.0	0.90	< 0.001	0.11	0.89
TTNDFD, % of total NDF		49.3	43.4	1.37	< 0.01	0.98	0.08
pdNDF, % of total NDF		75.5	70.2	0.86	< 0.01	0.44	0.01
pdNDF kd, % h^–1		3.6	3.4	0.27	0.57	0.69	0.99

Item		Treatments		SEM	p-value
Item		LI_95%	LI_Max	SEM	Trt	Per	Trt × Per
FCM, kg d^–1		18.4	16.9	1.18	0.02	0.02	0.63
Milk composition, %
	Fat	3.69	3.68	0.08	0.89	< 0.001	0.18
	Protein	3.12	3.35	0.09	< 0.01	0.06	0.86
	Lactose	4.49	4.38	0.04	< 0.01	0.02	0.96
	MUN, mg dL^–1	13.4	12.2	0.42	< 0.01	< 0.01	0.03
BW change, kg¹ 1 Data from Congio et al. (2018).		0.44	0.55	0.36	0.61	< 0.01	0.61

Item		Treatments		SEM	p-value
Item		LI_95%	LI_Max	SEM	Trt	Per	Trt × Per
Total VFA, mM		64.99	55.19	3.403	0.04	0.01	0.08
VFA, mol 100 mol^–1
	Acetate	68.02	68.85	0.232	0.01	< 0.001	0.28
	Propionate	16.90	16.53	0.202	0.20	< 0.001	0.97
	Butyrate	11.44	10.87	0.127	< 0.01	0.06	0.95
	Valerate	1.04	0.98	0.017	< 0.01	< 0.01	< 0.001
	Isobutyrate	1.05	1.06	0.029	0.94	< 0.001	0.10
	Isovalerate	1.53	1.47	0.061	0.47	< 0.001	0.04
Acetate: propionate ratio		4.04	4.18	0.062	0.12	< 0.001	0.88
Microbial synthesis, g d^–1		1312.3	1217.8	80.41	0.36	< 0.01	0.96
Microbial efficiency, g kg^–1		126.7	120.1	7.49	0.53	< 0.001	0.49

Item		Treatments		SEM	p-value
Item		LI_95%	LI_Max	SEM	Trt	Per	Trt × Per
NE_L intake, Mcal d^–1		25.4	20.4	0.95	< 0.01	0.33	0.01
Energy partitioning, Mcal d^–1
	NE_L maintenance	9.2	5.5	1.66	0.07	0.69	0.06
	NE_L BW gain	3.4	3.1	1.57	0.87	< 0.01	0.63
	NE_L activity	0.95	0.91	0.025	0.16	0.54	0.93
	NE_L milk	12.1	11.2	0.58	0.05	0.06	0.92
Efficiency of energy use, %		50.4	58.1	3.18	0.03	0.75	0.06

Item	Treatments		SEM	p-value
Item	LI_95%	LI_Max	SEM	Trt	Per	Trt × Per
Urine N, g d^–1	201.7	180.6	9.56	0.04	< 0.01	0.69
Fecal N, g d^–1	155.3	142.0	5.01	0.03	0.98	0.65
NUE, %	16.8	16.1	0.99	0.54	< 0.001	0.87
N excretion intensity, g N kg milk^–1	19.9	21.9	1.18	0.03	< 0.001	0.59

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[1] *Corresponding author <gcongio@gmail.com>