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SUMMARYThe objective of the present study was to determine the concentration of fatty acids (FA) in the milk of Chinampo cows (Bos taurus), with emphasis on conjugated fatty acids (CLA). A sample of 10 ml was collected from 32 cows that had been lactating at least 60 days, but not longer than 120 days. A group (FV, n = 25) was free-ranged grazed on green fresh pasture (sarcocaulescent shrub semiarid area, La Paz, B.C.S.) during the wet season (SeptemberOctober 2009). The second group (HA, n = 7) was penned and fed fodder of alfalfa hay. The FA analysis revealed that there were less saturated FA in the FV group (P < 0.05). This difference was basically explained by the lesser content of 14:0, 15:0, and 16:0. The concentration of total monounsaturated FA was higher in FV (P < 0.05), given by the higher concentration of 18:1 n-9 and 18:1 n-7 trans-11. The total concentration of polyunsaturated FA was similar for both groups, but there was a higher concentration of 18:2 n-6 and 18:2 n-7 cis-9 trans-11 in the FV group (P < 0.01). The proportion of omega-3 FA was similar for both groups, but the concentration of omega-6 FA was higher in FV (P < 0.01), resulting in an increased omega-6:omega-3 ratios in the FV group compared to HA. It is concluded that Chinampo cows left to free-range on fresh green pasture reduced the total saturated FA of medium chain length, and increased the concentration of 18:1 n-9, 18:1 n-7 trans-11 and 18:2 n-7 cis-9 trans-11, a benefit for human health. However, an increase in omega-3 FA is desirable in the milk of free-ranging cows in the area.Palabras clave: vacas, nutrición, ácidos grasos, leche.

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Efecto de la suplementación de grasas sobre características productivas, tasas de preñez y algunos metabolitos de los lípidos en vacas para carne en pastoreoEffect of fat supplementation on productive traits, pregnancy rates and some lipid metabolites in beef cows on range SUMMARYIn order to evaluate the effect of a supplementation with two sources of fat in beef cows on range, 120 adult females in two herds were used. One herd of 60 Charolais cows (A) and another herd of 60 Chinampo cows (B, local Creole) were fed (2 kg/cow/day) isoproteic concentrates (PC = 16.6%) with a similar content of ME (aprox. 3.0 Mcal/kg) during a 90 days period (initiating 30 days precalving), respectively. In each herd, cows were randomly assigned into three groups. The control groups (TE; n = 20) were fed a supplement containing 3.2% of fat. Another group of 20 cows per herd was fed a supplement containing additional fat (13.2%) as calcium soaps of fatty acids (JCAG). The third group (SB) within each herd (n = 20) was fed a supplement containing 13.4% of fat (bovine tallow). Orthogonal contrasts (TE vs SB + JCAG and SB vs JCAG) were used to compare the means of non categorical variables. In herd A, milk yield was greater at 35 (P < 0.05) and 90 (P < 0.01) days of lactation in cows fed supplemental fat (SB + JCAG) than TE treatment. In both herds, weight of calves at 35 and 90 days of age, body weight and body condition score of cows at 90 days postcalving were higher in SB + JCAG than TE (P < 0.01). Serum cholesterol, high density lipoproteins, low density lipoproteins and triglycerides were greater in cows supplemented with fat (SB + JCAG) than TE treatment (P < 0.01). Pregnancy rates in both herds were higher in fat supplemented cows (P < 0.05). We concluded that additional fat incorporated in a range supplement during pre-and postpartum periods increased milk yield, growth of calves, body weight and condition score of cows, lipid metabolites and pregnancy rates.Palabras clave: vacas para carne, pastoreo, suplementación, grasa.

RESUMEN. El objetivo fue caracterizar el crecimiento de vacas Siboney con modelos no lineales (MNL) y los efectos de los parámetros de crecimiento sobre la edad al primer empadre (EPE). Se analizaron 10 064 datos de peso vivo de 2 021 vaquillas. Los MNL evaluados fueron logístico, Gompertz, Brody y Bertalanffy. La selección del mejor modelo fue con el coeficiente de determinación, criterios Akaike y Bayesiano. Con regresión lineal de orden cuadrático, la EPE se analizó con respecto al peso a 24 meses (P24M), tasa de crecimiento (TAC), peso al punto de inflexión (PPI) y madurez a los 24 meses (GM24). El MNL de mejor ajuste fue Bertalanffy; con peso asintótico de 514.3 kg, punto de inflexión a los 5.93 meses con 152.4 kg. Del análisis de regresión: EPE = (0.026102)(P24M)-(0.0000636)(P24M 2), EPE = (23.86)(TAC)-(59.17)(TAC 2), EPE = (0.0796)(GM24)-(0.000675)(GM24 2), EPE = (0.0299)(PPI)-(0.00009506)(PPI 2), todos diferente de cero (p < 0.01).