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Background
It is estimated that 50% of the cows in extensive beef systems do not receive adequate nutritional management, and this is a main reason for low fertility rates in tropical herds [1].
Among the factors affecting the reproductive performance of beef cattle, nutrition has perhaps the highest impact [2]. Supplementation to grazing animals is a practice that can be adopted under tropical conditions to increase animal performance. Because the last third of gestation usually coincides with the dry season and, consequently, with low quantity and quality of forage, producers in tropical conditions, as in Brazil, are usually oriented to supplement pregnant cows for the last 90 d of gestation.
The objective with this experiment was to study whether this period of supplementation could be reduced to avoid extra labor and, consequently, reduce feeding costs. Therefore, we conducted a study to evaluate the effects of different supplementation strategies for pregnant beef cows in the last third of gestation, receiving supplementation for 90, 60 or 30 d pre-partum.
Methods
The two experiments were conducted at the Department of Animal Science - Universidade Federal de Viçosa, Brazil, from July to December 2012. All animal care and handling procedures were ethically standardized and approved by the Animal Care and Use Committee of the Universidade Federal de Viçosa, Brazil.
Experiment 1 - Performance
Thirty-five multiparous, at an average age of five years old, single pregnant Nellore cows with 491.88 ± 55 kg of body weight (BW), a body condition score (BCS) of 4.7 ± 0.58 and 200 ± 15 d of gestation were used. Cows were housed in an experimental area of Brachiaria decumbens divided into four paddocks of 5.0 ha each, and had unlimited access to water, feeders and mineral salt (8.7% calcium, 9.0% phosphorous, 18.7% sodium, 9.0% sulfur, 2,400 mg/kg of zinc, 800 mg/kg of copper, 1,600 mg/kg of manganese, 40.0 mg/kg of iodine, 8.00 mg/kg of cobalt and 8.16 mg/kg of selenium). To avoid any effects of paddock on the responses, treatments were rotated among paddocks every 10 d.
Four strategies were evaluated: 30-d - cows received 3.0 kg/d of supplement beginning 30 d prior to calving; 60-d - cows received 1.5 kg/d of supplement beginning 60 d prior to calving; 90-d - cows received 1.0...