Effects of bypass fat on energy balance, milk production and reproduction in grazing crossbred cows in the tropics Carlos Aguilar-Pérez a,b, ⁎, Juan Ku-Vera a , Philip C. Garnsworthy b a Departamento de Nutrición Animal, Facultad de Medicina Veterinaria y Zootecnia, Universidad Autónoma de Yucatán, Apdo. Postal 4-116, Mérida Yucatán, Mexico b School of Biological Sciences, University of Nottingham, Sutton Bonington Campus, Loughborough LE12 5RD, UK article info abstract Article history: Received 12 September 2007 Received in revised form 27 May 2008 Accepted 30 May 2008 This study was designed to investigate the effects of supplementation with a calcium soap of long chain fatty acids (ByFat ®) on energy balance, milk production and reproduction of suckling crossbred cows, grazing star grass in the tropics. ByFat was supplied at 1.8% of estimated dry matter intake (3% of live weight). Twenty-four cows were used in a 2 × 2 × 2 factorial design from calving to 98 days postpartum. Treatments were control (C) vs. ByFat, dry vs. rainy season, and primiparous vs. multiparous cows. There was no interaction between season or parity and diet. Milk yield was not different between diets (12.1 kg/day for ByFat, 11.7 kg/day for C), although cows fed ByFat were more persistent and lost body condition at a faster rate than control cows. ByFat had no effect on reproductive performance, possibly due to reduced grass DM intake (8.80 vs. 10.70 kg/day), which resulted in no treatment difference in energy balance or metabolic hormones. It was concluded that supplementation with a calcium soap of long chain fatty acids is not effective for improving energy balance or reproductive performance in low-merit grazing cows in the tropics. © 2008 Elsevier B.V. All rights reserved. Keywords: Energy balance Milk production Reproduction Crossbred cows Tropics 1. Introduction Grazing is the basis for ruminant feeding in the tropics and low grass quality and availability, especially during the dry season, are the main constraints for animal production and reproduction in these areas (García, 1991). Poor reproductive indices are typical in the tropics as a result of a long anoestrus postpartum (Anta et al., 1989). Negative energy balance (NEB) in early lactation has been suggested as a major factor affecting re-establishment of postpartum ovarian activity and fertility in high-merit dairy cows (Butler and Smith, 1989; Garnsworthy and Webb, 1999). There is now evidence that metabolic hormones, such as insulin, leptin and insulin-like growth factor I (IGF-I) play important roles in the control of ovarian follicular development and are likely to be important mediators of the effects of energy balance on cow fertility (Diskin et al., 2003; Webb et al., 2004). Nutritional alter- natives to overcome NEB include use of fats treated to be inert in the rumen and to be effectively absorbed in the lower tract (bypass fats). Such fats include calcium soaps of fatty acids, which have been extensively studied in dairy (Staples and Thatcher, 2005) and beef cows (Funston, 2004), but not in the so called “dual-purpose” crossbred cows, which are predo- minant in the Latin American tropics. Dual-purpose systems have been established in the tropics as a flexible way to produce milk and beef in the same herd, using local resources. These systems are important in many developing countries, not only because milk contributes to improvements in the human diet, but also for promoting employment and better quality of life for farmers (Boden and Anderson, 1998). Crossbred cows in the tropics have comparatively lower nutrient requirements and different partition of nutrients than high merit dairy cows (Chilliard, 1991). Because of these differences, crossbred cows would be expected to show greater response to additional energy in early lactation, improving their energy status and hence reproductive performance. Few studies of fat supplementation under Livestock Science 121 (2009) 64–71 ⁎ Corresponding author. Tel.: +52 999 9 42 32 00; fax: +52 999 9 42 32 05. E-mail addresses: caperez@uady.mx (C. Aguilar-Pérez), Phil.Garnsworthy@nottingham.ac.uk (P.C. Garnsworthy). 1871-1413/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.livsci.2008.05.023 Contents lists available at ScienceDirect Livestock Science journal homepage: www.elsevier.com/locate/livsci