Objectives were to evaluate the use of qualitative on-farm milk P4 measurements to predict non-pregnancy in lactating dairy cows. Jersey cows (n = 752) from 2 herds were subjected to timed AI (d −8 GnRH, d −3 and −2 PGF_2α, d 0 GnRH and AI). Milk was sampled on d −3, 0, 7, and 28 relative to AI. Samples were exposed to a lateral flow test (LFT) strip and classified into 3 groups: G1 = test line not visible or lighter than reference; G2 = test line similar to reference; G3 = test line darker than reference. Based on previous work, these groups indicate milk P4 concentrations of 17.1, 5.8 and 0.7 ng/mL, respectively. Pregnancy was diagnosed on d 34 and 62 after AI. Data were analyzed by multivariable logistic regression and orthogonal contrasts were built (C1: G1+G2 vs. G3; C2: G1 vs. G2). Proportions of cows in G1, G2, and G3 at each sampling point are depicted in the table below. Pregnancy per AI (P/AI) in G3 cows on d −3 was smaller (P < 0.01) compared with G2 and G1. Cows in G1 and G2 at AI had smaller (P = 0.01) P/AI than those in G3. Cows in G3 on d 7 had smaller (P < 0.01) P/AI compared with cows in G2 and G1. No difference was observed between G1 and G2 cows on d −3, 0, and 7. Cows in G3 on d 28 had the smallest (P < 0.01) P/AI followed by herdmates in G2 then G1. Pregnancy loss (PL) tended to be greater (P = 0.07) for cows in G2 at AI compared with G3. Milk P4 group on d 7 tended (P = 0.09) to influence PL, which was lowest in G2, followed by G1 then G3. Cows in G3 on d 28 had greater (P < 0.01) PL compared with G2 and G1. On-farm milk P4 data can be used to predict non-pregnancy and potentially allow for early resynchronization. Table 1. Pregnancy per AI [Adj. % (no.)] and pregnancy loss [Adj. % (no.)]

Item	Milk P4		P-value
	G1		G2	G3	Group	C1	C2
d −3
% total	51.8	21.8	26.4
P/AI d 62	37.2 (387)	38.3 (159)	17.4 (196)		<0.01	<0.01	0.80
PL	8.0 (161)	7.3 (67)	14.1 (41)		0.43	0.20	0.86
d 0
% total	1.6	10.0	88.4
P/AI d 62	0.0 (12)	15.6 (74)	34.8 (656)		<0.01	0.01	0.53
PL	—	21.3 (16)	8.0 (254)		0.07	—	—
d 7
% total	52.3	30.5	17.2
P/AI d 62	37.7 (387)	34.0 (225)	12.0 (128)		<0.01	<0.01	0.36
PL	10.6 (166)	3.4 (83)	17.3 (19)		0.09	0.10	0.06
d 28
% total	52.9	11.9	35.2
P/AI d 62	59.4 (379)	9.2 (86)	0.8 (258)		<0.01	<0.01	<0.01
PL	7.1 (246)	8.4 (9)	74.6 (7)		<0.01	<0.01	0.87