Abstract #98

Section: Production, Management and the Environment
Session: Production, Management & the Environment I
Format: Oral
Day/Time: Monday 11:30 AM–11:45 AM
Location: 324

# 98
Herd contextual effect modulates the relationship between cow milk yield and reproductive performance.
R. Rearte^*1,3, S. LeBlanc⁵, R. de la Sota^2,3, S. Corva¹, I. Lacau-Mengido^4,3, M. Giuliodori⁶, ¹Cátedra de Higiene, Epidemiología y Salud Pública, Facultad de Ciencias Veterinarias – Universidad Nacional de La Plata (FCV–UNLP), La Plata, Argentina, ²Cátedra y Servicio de Reproducción Animal, FCV–UNLP, La Plata, Argentina, ³Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Buenos Aires, Argentina, ⁴Instituto de Biología y Medicina Experimental–CONICET, Buenos Aires, Argentina, ⁵Department of Population Medicine, University of Guelph, Guelph, ON, Canada, ⁶Cátedra de Fisiología, FCV–UNLP, La Plata, Argentina.

Some studies have evaluated the relationship between milk yield and reproductive performance but without considering the multilevel nature of the data. So, our objective was to evaluate the effect of milk production at cow and herd levels and their interaction on the odds of pregnancy at the cow level; that is, to test for a herd contextual effect modulating the cow's probability of pregnancy. Data from 657,968 lactations started from 2001 to 2012 from 677 Argentinean dairy herds were included in the study. A mixed logistic regression model was fit to assess the odds of pregnancy by 100 DIM (P100) with fixed effects of cow milk yield to 80 DIM (CM80), quartiles of herd mean CM80 per herd-year (HM80) and their cross-level interaction. Herd-year was considered as a random effect. CM80 had a significant negative effect on P100 (OR = 0.95). Thus a cow belonging to a top quartile milk yield herd with CM80 1 SD (400 kg) over their HM80 had 5% lower odds of P100. The same increment in milk yield in cows from the lowest quartile of herd yield was predicted to have 1% lower odds of being pregnant by 100 DIM. An average producing cow in a top quartile yield herd had 5% greater odds of P100 than an average producer in a bottom quartile herd (Table 1). The herd contextual effect explained 4% of the observed variation in P100. The relationship between cow milk yield and reproductive performance is statistically significant but of small magnitude, and varies among herds: the association is negative in high-producing herds and positive in low-producing herds. Table 1. Logistic regression model of the effect of milk yield to 80 DIM at cow and herd levels on odds of pregnancy by 100 DIM

		Odds ratio	95%CI
CM80¹		0.95	0.95–0.95*
HM80²	Top	1
	Upper Middle	0.97	0.96–0.98*
	Lower Middle	0.96	0.95–0.98*
	Bottom	0.95	0.93–0.96*
CM80×Top		1
CM80×Upper Middle		1.00	0.94–1.06
CM80×Lower Middle		1.02	1.01–1.03*
CM80×Bottom		1.04	1.03–1.05*

¹CM80 = overall milk yield per cow by 80 DIM centered on HM80 (SD= 400 kg). ²HM80 = mean CM80 per herd-year divided into quartiles. *P < 0.05.

Key Words: contextual effect, reproduction