1. SUCKLING CONTROL IN PRIMIPAROUS BEEF COWS UNDER RANGE CONDITIONS G. Quintans, C. Jiménez de Aréchaga and A.I. Vázquez National Institute of Agricultural Research (INIA), Ruta 8 Km 281, 33000, Uruguay. Introduction Reproductive efficiency is key to the achievement of high productivity in cow-calf production systems. One of the major causes of poor reproductive efficiency in beef suckled cows is an extended postpartum anoestrous interval which, under range conditions can exceed 120 days (Quintans and Vázquez, 2002). Suckling and nutrition are generally recognised as two major factors controlling the length of the post-partum anovulatory period (Short et al., 1990). In a preliminary study, ten days of calf removal had similar benefit to nose plates on ovarian cyclicity but on the former, calves presented higher daily live weight gain without compromising maternal bond (Quintans et al., 2003). The objective of this study was to evaluate the effects of different suckling control techniques on reproductive performance of primiparous beef cows and on calves performance. Materials and Methods Forty Angus x Hereford three years old primiparous cows were used in the study. They calved with (mean ± s.e.m) 359 ± 3.4 kg of live weight and 4.7 ± 0.05 units of body condition (BCS, 1= thin, 8= fat) and they were assigned to four treatments at 67±0.87 days postpartum (Day 0): calves weaned from their mothers (W; n=10) calves fitted with nose plates during 14 days remaining with their mothers (NP; n=10) calves were completed removed from their mothers for 10 days (CR; n=10) calves remained with and had free access to their mothers (C; n=10). All cows were in anoestrous on Day 0. The breeding period lasted 2 months, started on Day 0 and it was by natural service using one bull of proven fertility. Blood samples were collected twice weekly from Day -7 until the end of the breeding period (Day 60) for progesterone analysis. Cow live weight (LW) and BCS were recorded at calving and at biweekly intervals. Results Table 1. Interval from onset of treatments (Day 0) to first normal oestrous cycle (OC) and percentage of ovulated cows. Conclusions Cows calved in good body condition and the use of suckling control such as early weaning or calf removal for ten days was effective to advance the first oestrous cycle and tended to increase the number of cows ovulating. Ten days of calf removal did not compromise either the maternal bond or the normal growth of calves. During the first 14 days after the onset of treatments, calves in W and CR lost 0.42±0.09 and 0.56±0.09 kg/d respectively, while calves in NP gained 0.28±0.09 kg/d and in C, 1.05±0.09 kg/a/d (P<0.05). The mean daily LW gain of calves during the total period of the experiment was lower (P<0.05) in W (0.53±0.08 kg/d) than in NP (0.78±0.08) and was intermediate but not different for calves in CR (0.76±0.08) and C (0.68±0.08). References Quintans and Vázquez, 2002 Proc. of the Sixth International Symposium in Domestic Ruminants, Crieff, Scotland (A65) Quintans et al., 2003 Proc. of the IX World Conference on Animal Production, Porto Alegre, Brasil, pp 219. Short et al, 1990, Journal of Animal Science 68:799-816