The objective of this experiment was to determine the effects of 500 mg of supplemental β-carotene on milk protein production and rumen fermentation. Sixteen multiparous lactating Holstein cows were blocked by DIM and randomly assigned to 1 of 4 treatments in a replicated 4 × 4 Latin square design. Two diets were fed: 9% RDP diet that contained (DM basis) 9.2% grass silage, 38.6% corn silage, 11% alfalfa hay, 25.7% energy mix, 11% protein mix, 2.2% mineral-vitamin premix, 1.6% protected fat and 0.6% bloodmeal, and an 11% RDP diet that contained (DM basis) 9.2% grass silage, 38.4% corn silage, 12% alfalfa hay, 23% energy mix, 2.2% mineral-vitamin premix, 1.6% protected fat, 0.9% bloodmeal, 0.5% urea and 12.3% soybean meal. Five hundred mg of β-carotene was top-dressed daily. Previously, incubating β-carotene in rumen fluid resulted in lower NH₃-N and higher microbial protein, indicting it may increase utilization of NH₃-N, leading to increased growth of microbes. If found to be beneficial, dairy producers would supplement β-carotene and be able to reduce purchased protein costs, therefore increasing profitability. Limiting crude protein can also prove to be beneficial by reducing nitrogen loss to the environment. Results are shown in Table 1. In this study, β-carotene did not maintain milk protein production when supplemented to a 9% RDP diet, compared with an 11% RDP diet. Therefore, based on these data, it is not recommended to supplement β-carotene to lactating cows under these dietary conditions. Table 1.

Item	Treatment				SEM	P-value
9%RDP	9%RDP+BC	11%RDP	11%RDP+BC	RDP	BC	Int
DMI, kg/d	26.6	26.8	27.5	26.8	0.56	0.006	0.11	0.02
Milk yield, kg/d	43.4	44.0	44.4	43.6	1.62	0.32	0.56	0.02
ECM, kg/d	49.4	49.8	50.4	49.5	1.34	0.38	0.59	0.16
4% FCM, kg/d	42.2	42.6	42.9	42.3	1.11	0.67	0.80	0.31
ECM/DMI, kg/d	1.85	1.85	1.84	1.85	0.04	0.90	0.80	0.79
Fat, %	3.91	3.91	3.84	3.91	0.14	0.34	0.39	0.40
Fat, kg/d	1.66	1.67	1.67	1.66	0.04	0.86	0.91	0.65
Protein, %	3.37	3.36	3.42	3.43	0.06	<0.0001	0.75	0.67
Protein, kg/d	1.44	1.45	1.50	1.47	0.04	0.0003	0.18	0.04
SNF, %	9.07	9.06	9.12	9.10	0.06	0.008	0.18	0.85
SNF, kg/d	3.92	3.96	4.04	3.94	0.13	0.09	0.29	0.01
MUN, mg/dL	8.38	8.62	12.81	12.75	0.33	<0.0001	0.54	0.33