Background: Genetic variability of populations is essential for their genetic conservation and improvement, and genealogy analysis is a useful tool to estimate it. Objective: To determine genetic variability and inbreeding levels in Tropical Milking Criollo (LT). Methods: This study analyzed the genealogy of LT using the ENDOG software. Registration certificates of 3,427 LT animals born between 1945 and 2013, and from 608 born between 1950 and 2013 were used. Two populations were defined: the first one with all registered animals (PLT), and the second with nucleus animals (PCP). Results: Estimates for PLT and PCP were: founders 890, 114; ancestors 855, 102; effective founders 111, 43; effective ancestors 72, 26, and effective population size 68.1 and 64.6, respectively. Inbreeding coefficients were 4.32 and 3.48% for the highest genetic integrity index; and the average relatedness (AR) coefficients were 1.19 and 5.55 for PLT and PCP, respectively. Genealogy depth was shallow in both populations, with full equivalent generations of 2.00 and 3.53. Global generation intervals were about seven years. Conclusions: The LT population is not endangered and its genetic improvement program should continue.

Milk production under grazing conditions is carried out in the intertropical region of hot climates due to its low production cost. The Tropical Milking criollo breed (LT) is characterized by its hardiness in this region, and its high milk quality and cheese yield. Grazing supplementation can increase milk production; however, it can also change its chemical composition. The effect of supplementation with commercial feed in LT cows was evaluated concerning the quantity and physicochemical traits of their milk by lactation. The treatments used were feeding based only on grazing para grass (Brachiaria mutica) and grazing plus 1 kg supplementation with 20 % protein commercial feed for every 5 kg of milk produced daily. The daily milk production per cow increased from 5.82 ± 0.18 to 7.10 ± 0.18 kg due to supplementation. Dry matter intake was similar in both treatments. The concentration of fat, protein, lactose, non-fat, and total solids did not suffer modifications (p > 0.05), but the daily production of each component per cow increased in animals supplemented due to the multiplicative effect of the amount of milk. The number of calvings affected milk production, fat, and ureic nitrogen in milk, and somatic cell count (p ≤ 0.05). It can be concluded that the supplementation used in this study was enough to increase milk production by 22 %, without modifying its chemical composition.

El objetivo de este estudio fue estimar las frecuencias genotípicas y alélicas de las variantes A1 y A2 de la b-caseína y determinar su relación con la producción de leche por lactancia (PL305) en el bovino Criollo Lechero Tropical (CLT). Se recolectaron muestras de sangre de 151 hembras; a partir del ADN un segmento del gen de la b--caseína se amplifico por PCR y se genotipificó por digestión con endonucleasa de restricción. Las frecuencias genotípicas para A1A1, A1A2 y A2A2 fueron 0,09, 0,78 y 0,13 (p£0,05) y las frecuencias alélicas para A1 y A2 fueron 0,48 y 0,52 (p>0,05). Las frecuencias genotípicas y alélicas de la b-caseína en el ganado CLT fueron similares a la raza Holstein. No se presentó efecto (p>0,05) de los genotipos de la b-caseína en PL305; medias de mínimos cuadrados fueron 1124±93, 1130±152 y 1257±74 para A1A1, A1A2 y A2A2. Continuarán estudios simultáneos de los genotipos para la b-caseína, k-caseína y b-lactoglobulina en la producción y composición de la leche de las vacas CLT.