Animal Reproduction Science 73 (2002) 159–168 Postpartum anoestrus is reduced by increasing the within-day milking to suckling interval in dual purpose cows P. Pérez-Hernández a , M. Garc´ ıa-Winder b , J. Gallegos-Sánchez b,∗ a Colegio de Postgraduados, Campus Veracruz, Veracruz, Mexico b Programa de Ganader´ ıa, Instituto de Recursos Genéticos y Productividad, Colegio de Postgraduados, Km. 36.5, Carretera Federal México-Texcoco Montecillo, Texcoco 56230, Estado de México, Mexico Received 5 March 2001; received in revised form 20 June 2002; accepted 5 July 2002 Abstract In the dual purpose milk/beef production system practised in Mexico, the cows are milked once a day in the presence of the calf and then the calf suckles for the next 1–8 h. It was hypothesised that the imposition of milking and suckling in close succession contributes to the long postpartum anoestrous periods experienced by cows in this system. Therefore, separating these two inhibiting factors, by de- laying the start of suckling after milking would lower the impact on the hypothalamus–hypophysial axis and allow it to recover more readily. This hypothesis was tested in two experiments using Bos taurus × Bos indicus cows which weighed 541.3 ± 12.7 kg (mean ± S.E.M.; experiment 1) and 537.8 ± 10.6 kg (experiment 2) and had a body condition score of 5–6 (scale 1–9) at calving. The cows and calves were maintained on separate pastures and the cows were offered concentrate supplements. Experiment 1 was conducted in the dry season and cows were offered a daily sup- plement of 3 kg of 17% crude protein (CP) concentrates plus 1 kg molasses per cow. Experiment 2 was conducted during the wet season and the nutritional supplement consisted of only 1 kg of 17% CP plus 1kg molasses per cow per day. Each experiment lasted for the first 100 days postpartum and the cows were not mated during this time. Cows were hand-milked once a day in the presence of the calf to stimulate milk let-down. In experiment 1, 30 multiparous cows were assigned at random by date of calving, to one of three treatments: controlled suckling (CS; n = 10), delayed suckling (DS; n = 10) and prolonged delayed suckling (PDS; n = 10). The suckling period on all treatments lasted for 2 h and started at 0 h (CS), 4 h (DS) or 8 h (PDS) after milking. In experiment, 34 multiparous cows were assigned at random to three treatments: CS (n = 11), PDS (n = 11) or prolonged delayed short suckling (PDSS; n = 12), in this last treatment the suckling period was reduced to only 30 min. In both experiments, one quarter of the udder of cows on treatment CS was not milked but all the udder was milked on the other treatments. Body weights of cows and calves and cow milk yields were recorded. Weekly blood samples were collected for plasma progesterone ∗ Corresponding author. Tel.: +52-5804-5979; fax: +52-5952-0279. E-mail address: gallegos@colpos.colpos.mx (J. Gallegos-S´ anchez). 0378-4320/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0378-4320(02)00147-1