Studies on 59 first four completed lactations of 37 imported Jersey cattle in Western Nigeria showed that their mean lactation length (248-4 days) and average milk yield (1257-4 kg) were inferior to those of their counterparts in other tropical countries. Lactation length following the birth of female calves was longer and milk production greater than those subsequent to male births but the differences were small (P > 0-05). Peak milk production was reached at the third lactation. Influence of season of calving on lactation length was slight; mean milk yield was significantly highest (P < 0-05) among October-December calvers, while the July-December period appeared to be more propitious for milk production. The regression coefficients of lactation length and milk yield on age of dam at calving were negative and unimpressive (P > 0-05), being -2-108 ±2-661 and -19-106 ± 17-130 respectively. Analysis of variance for weight of milk with lactation length as a covariate revealed that although lactation length was a major (P < 0-001) determinant of milk production, both the period of calving and lactation number had recognizable bearing on milk yield. All interaction effects were trivial (P > 0-05).

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The birth weight of Friesian and Holstein calves in Western Nigeria was generally lower than that recorded in other tropical countries. This was attributed to a low plane of nutrition during pregnancy and a consequent reduction in length of gestation of the dams. On average, pure-bred Friesian, pure-bred Holstein and Holstein x Friesian calves weighed 29-1, 30-8 and 30-7 kg respectively at birth. Male and female Friesian, Holstein and cross-bred calves averagely weighed 29-9 and 28-2 kg, 29-9 and 31-7 kg, and 32-6 and 28-8 kg respectively. Both the sex differences and breed x sex interaction were not significant.Positive correlation coefficients of 0-45, 0-38 and 0-26 found between calf birth weight and gestation length for the Friesians (P < 0-05), Holsteins (P < 0-01) and their crosses (P > 0-05) respectively suggest that gestation length had a significant effect on Friesian and Holstein calf birth weight. In many cases, parturition occurred 8-16 days earlier than expected.Calves born during the rainy season (April-September) were approximately 10% heavier at birth than those born during the dry season (October-March). The heaviest calves were born during the second half (July-September) of the rainy season. The breed x sex x season of calving interaction was significant for the Holstein breed (P < 0-05). Holstein male calves born during the rainy season were significantly (JP < 0-05) heavier than those born during the dry season. Friesian dams with three or more parities and Holstein dams that were 3 years old and above produced calves that were significantly (P < 0-01) heavier at birth than calves of younger dams.

The average of 160 intercalving periods of 47 imported and 33 home-reared purebred Holsteins in western Nigeria during 1967-75 was 628( + 45) days. The probable effects of seven variables including origin of cow, sex of calf, season of calving, age of cow at calving, parity, year of calving and calf birth weight were investigated through an intra-class linear regression model in which the first three factors were treated as qualitative and the last four as quantitative factors. Origin of cow, sex of calf and season of calving did not significantly affect calving interval. Effects of age at calving, parity, year of calving and calf birth weight were highly significant (P < 0-01) but individual effects adjusted for others were not quite significant. These four quantitative factors were, however, highly interrelated; simple correlations which varied between 0-33 and 0-86 were highly significant. Interactions between the two sets of factors were negligible.

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