Abstract One of the main problems to be solved in the commercial production of Nile tilapia is the use of large volumes of steroids during the sex reversion process. The precise evaluation of the time after hatching to start the steroid treatment could help reduce its use in the short-term. The main objective of this work was to evaluate the sex proportion and the growth obtained in a mixed population of Nile tilapia fed fluoxymesterone at different days (3, 6, 9, 12, and 15) after hatching. A one-factor experiment was designed, with the day, after hatching, that the fluoxymesterone (5 mg) began to be supplied as factor (3, 6, 9, 12 and 15). Initial stocking density was 0.4 larvae L-1. All groups were evaluated by triplicate. Steroid treatment finished at day 25 after larvae stocking. Squash technique was used to determine sex proportion at the end of the experiment (45 days). Growth evaluation was performed based on wet weight and total length obtained from each biometry. The results showed the highest percentage of males (92%) in the fish fed fluoxymesterone from day three after hatching. No anabolic effect on growth was observed in any steroid-treated fish. A negative effect on growth was observed after withdrawal of the steroid in the fish that were fed fluoxymesterone for the longest time. These results show that it is necessary to supply the fluoxymesterone prior to the onset of sex differentiation (5-6 days after hatching) to achieve a permanent reversion of the sex differentiation process.

Both growth and milk production in heifer calves are stimulated by a long photoperiod, though this has not been proven yet in subtropical areas. To evalúate the effect of 16 hr of light (L16) on calves suckling (LAC) and/or pre-pubertal stages (PP) in a subtropical area, 325 calves (36 ± 0.4 kg) were randomized into two groups: L16 or natural photoperiod (LNAT). At seven months of age, 198 of these calves (195 ± 2 kg) were randomly assigned to L16 or LNAT. The following were determined at the beginning and end of LAC and PP stages: Weight (PC, kg), height (AC, cm), thickness of back-fat (GG, cm) and of Longissimus dorsi (LD, cm); pelvic area (AP, cm²), body condition (CC), age at first estrus, mammary gland depth (PGM, cm) and width (AGM, cm). The statistical analysis was made using ANOVA for a 2 X 2 factorial arrangement. At the end of LAC, GG was lower and LD greater in L16 animals, while, at the end of PP, L16 heifer calves had a greater PC (260 ± 3 vs 250 ± 3), AP (166 ± 1.2 vs 153 ± 1.2), LD (3.18 ± 0.04 vs 2.90 ± 0.04), and AGM (2.41 ± 0.02 vs 2.21 ± 0.02), but a lower GG (0.114 ± 0.003 vs 0.139 ± 0.003) as compared to LNAT heifer calves. More L16 animals (67 %) presented estrus versus LNAT (38 %) and the age of the rst estrus was lower in L1 (278 ± 2 vs 288 ± 2 d). Consequently, exposure to L16 during LAC promotes lean growth while lean growth and a bigger pelvis and mammary gland are prompted during PP. Hence, it may be concluded that PP heifer calves exposed to L16 have a higher production potential and lower risk of dystocia than LNAT animals.

En vaquillas el fotoperiodo largo estimula el crecimiento y la producción láctea, pero esto no se ha probado en áreas subtropicales. Para evaluar el efecto de 16 h luz (L16) durante la lactancia (PW) y/o la etapa prepuberal (PP) en un área subtropical, 325 becerras (36 ± 0.4 kg) fueron distribuidas a: L16 ó fotoperiodo natural (LNAT). A los siete meses de edad, 198 de esas becerras (195 ± 2 kg) se asignaron aleatoriamente a L16 y LNAT. Al inicio y final de PW y PP se determinó: peso (PC, kg), altura (AC, cm), grosor de grasa dorsal (GG, cm) y del músculo Longissimus dorsi (LD, cm); área pélvica (AP, cm²), condición corporal (CC), edad al primer estro, profundidad (PGM, cm) y anchura (AGM, cm) de la glándula mamaria. El análisis estadístico fue por ANDEVA para arreglos factoriales 2X2. Al final de PW, GG fue menor y LD fue mayor en L16. Al nal de PP, las vaquillas L16 tuvieron más PC (260 ± 3 vs 250 ± 3), AP (166 ± 1.2 vs 153 ± 1.2), LD (3.18 ± 0.04 vs 2.90 ± 0.04) y AGM (2.41 ± 0.02 vs 2.21 ± 0.02), pero menos GG (0.114 ± 0.003 vs 0.139 ± 0.003) que las de LNAT. Más vaquillas L16 (67 %) presentaron estro que LNAT (38 %) y la edad al primer estro fue menor en L16 (278 ± 2 vs 288 ± 2 d). Consecuentemente, la exposición a L16 durante PW promueve crecimiento magro y durante PP induce crecimiento magro, mayor pelvis y glándula mamaria. Se concluye que vaquillas PP expuestas a L16 tienen mayor potencial productivo y menor riesgo de distocia que las LNAT.

The purpose was to estimate the forage yield in genotypes of maize with forage potential in response to different sown densities. A first essay (Expl) was made in November 2007 to February 2008, a second study (Exp2) was made in 2009 in Loma Bonita, Oaxaca, Mexico, evaluating the genotypes H-520, HE-1A17, HE-2A15, V-556AC, VS-536, A7573, H-564C and a native variety, in three densities and four replications. Treatments were the combination of genotypes and densities, in a randomized block design. In average of densities native variety had the highest plant height (Exp1=247, Exp2=216 cm), total leaf area (Exp1=5834, Exp2=7516 cm²), stem diameter (Exp1=6.6 cm) and forage yield (Exp1=44.3 t ha-1). In the second experiment H-564C genotype had the major stem diameter (8.2 cm), number of leaves upper the ear (7.3), ear weight with leaves (251.7 g), ear weight without leaves (148.7 g), ear height (18.0 cm), ear diameter (13.2 cm), yield per plant (564.7 g) and per hectare (36.6 t ha-1). In average of genotypes, the density of 83,333 plants exceeded in forage yield (Exp1=35.8; Exp2=37.3 t ha-1) to the density of 62,500 (Exp1=32.1; Exp2=32.1 t ha-1) and 50,000 plants ha-1 (Exp1=23.1; Exp2=25.5 t ha-1). Combined analysis indicated that the native variety produced more forage than hybrids. For density, the maximum yield by genotype was established at 83333 plants ha-1 the native variety; it produced 57.1 t ha-1 of forage, above the maximum yield of H-520 and the control VS-536 in that density.

El objetivo fue estimar el rendimiento de genotipos de maíz con potencial forrajero en respuesta a densidades de siembra. El Exp 1 se hizo de noviembre de 2007 a febrero de 2008 y el Exp 2 se realizó en 2009, en Loma Bonita, Oaxaca, México, evaluándose los genotipos H-520, HE-1A17, HE-2A15, V-556AC, VS-536, A7573, H-564C y un criollo, en tres densidades y cuatro repeticiones. Los tratamientos fueron la combinación de genotipos y densidades, en un diseño de bloques al azar. En promedio de densidades, el criollo tuvo la mayor altura de planta (Exp1=247, Exp2=216 cm), área foliar total (Exp1=5,834, Exp2=7,516 cm²), diámetro de tallo (Exp1=6.6 cm) y rendimiento de forraje (Exp1=44.3 t ha-1). En el Exp2 el H-564C tuvo el mayor diámetro de tallo (8.2 cm), número de hojas arriba del elote (7.3), peso de elote con (251.7 g) y sin brácteas (148.7 g), longitud (18.0 cm) y diámetro de elote (13.2 cm), rendimiento por planta (564.7 g) y por hectárea (36.6 t ha-1). En promedio de genotipos, la densidad de 83,333 plantas ha-1 (Exp1=35.8; Exp2=37.3 t ha-1), superó en rendimiento de forraje a la de 62,500 (Exp1=32.1; Exp2=32.1 t ha-1) y 50000 plantas ha-1 (Exp1=23.1; Exp2=25.5 t ha-1). El análisis combinado indicó que el criollo produjo más forraje que los híbridos. Por densidad, el criollo a 83,333 plantas produjo 57.1 t ha-1 de materia verde, superando en esa densidad al H-520 y al testigo VS-536.

Densidad de siembra y crecimiento de maíces forrajeros. El objetivo de este trabajo fue identificar genotipos de maíz con potencial forrajero. Un primer trabajo (E1) se hizo de noviembre del 2007 a febrero del 2008, se evaluaron los genotipos H-520, HE-1A17, HE-2A15, V-556AC, VS-536, A7573 y un genotipo criollo, en tres densidades (50 000, 62 500, 83 333 plantas/ha) y cuatro repeticiones. El segundo ensayo (E2) (de enero a abril del 2009) se prescindió del HE-1A17 y HE-2A15, y se incluyó el H-564C. Los tratamientos fueron la combinación de genotipos y densidades, en un diseño de bloques al azar. Se hicieron análisis de varianza por experimento, el combinado y comparación de medias por Tukey (P≤0,05). El genotipo criollo en el promedio de densidades presentó la mayor altura de planta (E1=247, E2=216 cm), área foliar total (E1=5834, E =7516 cm2) e índice de área foliar (E =3,8; E =4,9), diámetro de tallo (E1=6,6; E2=7,5 cm) y rendimiento de forraje en E1=44,3 t/ha. En el segundo ensayo el H-564C (E2=36,6 t/ha) superó en rendimiento al testigo VS-536. En promedio de genotipos, la densidad de 83 333 plantas (E1=35,8; E2=37,3 t/ha), superó en rendimiento de forraje a la de 62 500 y 50 000 plantas/ha. Por densidad, el máximo rendimiento se tuvo con el genotipo criollo establecido a 83 333 plantas con 57 t/ha de matera verde. En promedio de densidades el mayor índice de área foliar se alcanzó con 83 333 plantas y el menor se obtuvo con 50 000 plantas/ha

Plant density and growth of forage maize genotypes. A study was conducted to identify genotypes of maize with high forage potential. The first experiment (E1) was conducted from November 2007 to February 2008, evaluating the genotypes H-520, HE-1A17, HE-2A15, V-556AC, VS-536, A7573 and a native variety, at three densities (50000, 62500 and 83333 plants/ha) and with four replications. In a second experiment (E2) (January to April 2009) the hybrids HE-1A17 and 2A15 were excluded and H-564C was added. The treatments were the combination of genotypes and sowing densities, in a randomized block design. Analysis of variance for each experiment were conducted, and combined and comparison of means, by Tukey (P≤0.05). In the first experiment, the native genotype, average of densities, had the highest plant height (E1=247, E2=216 cm), total leaf area (E =5834, E =7516 cm2) and leaf area index (E =3.8; E2=4.9), stem diameter (E1= 6.6; E2=7.5 cm) and forage yield (E1=44.3 t/ha). In the second experiment, H-564C (E2=36.6 t/ha) surpassed the control. The density of 83333 plants/ha, average of genotypes, surpassed in forage yield (E1=35.8; E2=37.3 t/ha) the density of 62500 plants and 50000 plants/ ha. For density, the maximum yield by genotype was established at 83333 plants/ha in the native variety, which produced 57 t/ha of forage. On average of the densities, the maximum leaf area index was achieved with 83333 plants/ha, and the lowest was obtained with 50000 plants per hectare.