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Gestation length and birth weight of 939 calves born at Santa Monica Experimental Station, Valenc,a, State of Rio de Janeiro, were studied. The calves were of six red and white Holstein-Friesian (HF) x Guzera (G) grades: 1/4, 1/2, 5/8, 3/4, 7/8 and =s 31/32.As the grade x sex interaction for birth weight was significant, data for each sex were analysed separately. A model, including the effects of grade, year-season of birth, grade x year season interaction and age of dam as a covariate, resulted in the following least-square means for the six grades in the above order (± s.e.): gestation length for cows carrying male calves, 290-0 (± 0-9), 281-2 (± 0-9), 285-3 (± 0-8), 278-8 (± 0-9), 280-5 (± 0-9) and 279-3 (± 0-9) days; gestation length for female calves, 287-5 (± 0-9), 280-1 (± 1-0), 285-3 (± 0-9), 274-5 (± 1-1), 279-2 (± 0-9) and 276-4 (± 1-0) days; birth weight for males, 34-6 (± 0-7), 28-6 (± 0-8), 34-2 (± 0-7), 32-4 (± 0-7), 35-0 (± 0-7) and 34-7 (± 0-7) kg; birth weight for females, 29-9 (± 0-6), 29-5 (± 0-7), 33-4 (± 0-6), 31-9 (± 0-7), 33-6 (± 0-6) and 32-6 (± 0-6) kg.Direct (g 1 ) and maternal (g M ) breed additive effects (HF -G), and direct (h 1 ) and maternal heterosis \h M ) were estimated. The estimates of g 7 , g M , h 1 and h M were respectively, for gestation length for male calves: -21-5 (± 1-9), 10-6 (± 2-1), 1-5 (± 1-8) and -1-6 (± 11) days; for gestation length, for female calves: -21-9 (± 2-1), 14-6 (± 2-5), 5-7 (± 2-1) and -1-5 (± 1-2) days; for birth weight of male calves: -3-5 (+ 1-5), 7-1 (± 1-8), -0-6 (± 1-5) and 1-0 (± 0-9) kg; and for birth weight of female calves: 4-7 (+ 1-4), 4-1 (± 1-6), 2-8 (± 1-3) and 2-0 (± 0-8) kg. The difference in birth weight between males and females was 4-7 (± 0-9) for G-sired calves and 0-8 (± 0-4) for HF-sired calves. 157

Burdens of ticks (Boophilus microplus) on young and adult females (heifers and cows) of six red-and-white Holstein-Friesian (HF) × Guzera (G) grades were assessed by counting the number of semi-engorged tick females on the right side of the animals. The HF grades were: ¼ ½ ⅝ ¾ ⅞ and ≥ Assessments of tick burdens of heifers were made on twelve occasions (357 observations on 193 animals). Cows were assessed six times (380 observations on 83 animals). Cows and heifers were in different pastures. Data were transformed to log10 (2 × count + 1) to normalize their distribution. Direct breed additive (g1, HF-G) and heterosis (hl) effects were estimated within dates of counting. Estimates of h1 on the log scale were not significant (P > 0·05) on 11 out of 12 dates for heifers and on five out of six dates for cows. Estimates of g1 for heifers varied from 0·505 (s.e. 0·492) to 2·376 (s.e. 0·345) (mean g1 = 1·575 (s.e. 0·096)), and estimates for cows varied between 1·009 (s.e. 0·203) and 2·293 (s.e. 0·219) (mean g1 = 1·416 (s.e. 0·080)). These results indicate the presence of important genetic effects on tick burdens. The means of the untransformed number of ticks per animal were, for the six grades in the above order, respectively: 44, 71, 151, 223, 282 and 501, for heifers, and 7, 19, 31, 64, 62 and 97 for cows.

Age and weight at puberty, age at first conception and number of services per conception of 90 females at Santa Monica Experimental Station, Valencia, State of Rio de Janeiro, were studied. The females were of six red and white Holstein-Friesian (HF) x Guzera (G) grades: 1/4, 1/2, 5/8, 3/4, 7/8 and =s 31/32.A model (1) including the effects of grade and season of birth resulted in the following least-squares means for the six grades in the above order (± s.e.): age at puberty0-2) and 1-3 (± 0-2).A second model (2) was fitted, including the effects of season of birth, direct (g 1 ) breed additive effects (HF -G) and heterosis effects (h l ). g 1 and h' were estimated by the partial regression coefficient of the traits studied on, respectively, the expected HF gene frequency and the expected heterozygosity of the females. The estimates of g 7 were not significantly different from zero except for age at first conception (g 7 = -102 (s.e. 46) days) (P < 0-05). Heterosis effects were significant for age at puberty (h' = -86 (s.e. 34) days), weight at puberty (h 1 = 44 (s.e. 17) kg) and age at first conception (h 1 = -119 (s.e. 37) days). These three estimates of h 1 amounted, respectively to -0106, 0151 and -0-132 of the estimated mean of the parental breeds.F-tests on the extra variation due to fitting model 1 after model 2 were not significant (P < 0-05) indicating that epistasis or other genetic effects, not included in model 2, were of little importance for the traits studied.