Certain aspects of the reproductive performance of zebu cattle in Cameroon

English: A study divided into two phases was carried out in order to assess the reproductive characteristics of the Ngaoundere Gudali cow of the Adamawa Highlands of Cameroon. In the first part of the study, records from 1966 to 1994 collected at the Wakwa Agricultural Research Centre and Animal Production Station were used to study productive and reproductive traits such as: the gestation length, the postpartum period, the age at first calving and calving intervals. The birth and weaning weights as well as the pre- and post-weaning growth rates of Gudali calves were also evaluated. The second part of the study consisted of a monitoring study conducted between November 2001 and May 2002 on the length of the oestrous cycle, duration of oestrus, the serum progesterone levels during the oestrous cycle, and the effect of two suckling and two weaning regimes on the postpartum ovarian activity of Ngaoundere Gudali cows. The General Linear Models procedures of SAS (1991) were used to statistically analyse the data. Due to the unbalanced nature of the data, Least Squares Means (LSM) were used and, whenever a significant effect was detected, the Tukey-Kramer test was used to evaluate the LSM. The overall least square mean for gestation length in Gudali cows was recorded as 293.4 ± 0.4 days (N = 697). The sex of the calf significantly (P<0.05) affected the length of gestation, with male calves being carried in utero approximately 3 days longer than their female counterparts (294.1 ± 1.2 and 291.1 ± 1.2 days, respectively). Birth weight tended to increase as the gestation period lengthened. The significant effect (P<0.01) of the month of service on gestation length could not be explained, as gestation length fluctuated from one year to another. The parity and age of the cow had no significant effect on gestation length. The mean duration of the open period (from calving to conception) was 267.7 ± 7.4 days, with a range of 24 to 749 days. Approximately 23.2% of the cows conceived within 90 days of calving and a total of 55.6% had conceived by 360 days. The distribution of the open period was bimodal, and could have been influenced by the seasonal availability of feed, or the long (6 months) mating season that allowed cows to calve during the following mating season. The calving to conception interval was significantly (P<0.001) affected by the month of calving and parity, but not by the sex of the calf. Cows calving at the end of the dry season/beginning of the rainy season (March and April) had the longest open period (and consequently the longest calving interval), compared to cows calving later (July to September). Cows on their first parity experienced a significantly longer mean postpartum (open) period (P<0.001) than older cows (320.7 vs. 200.2 days for first and 5th parity cows, respectively). The sex of the calf did not affect the duration of the postpartum period, although this period was on average 5 days longer following the birth of a male than a female calf. The overall average birth weight was 24.1 ± 2.8 kg (N = 3401), with male calves weighing approximately 1 kg more than their female counterparts (24.6 ± 0.1 and 23.7 ± 0.1 kg, respectively). Calves born during the rainy season were 0.84 kg heavier than those born during the dry season (24.5 ± 0.1 and 23.7 ± 0.2 kg, respectively). No significant effect of parity was recorded on birth weight, although the females calving for the first time tended to have lighter calves than older cows. A year effect on birth weight was detected, but no definite trend towards an increase or a decrease was noted over the years. This year effect was attributed to the large variation in environmental factors (rainfall and nutritive value of the pastures) experienced by the animals. The average weaning weight of Ngaoundere Gudali calves was recorded as 149.4 ± 1.4 kg (N = 1827), with male calves weighing 6.1% more than the female calves at weaning (157.0 ± 2.8 and 148.0 ± 2.7 kg, respectively). There was a 20.4% weaning weight advantage of dry season born calves over those born during the rainy season (166.6 ± 4.5 and 138.4 ± 1.9 kg, respectivelv), despite the fact that the former were born significantly lighter. There was also a significant (P<0.001) year effect on weaning weight, with no evident trend over the years. This could be due to the variation in environmental conditions (inconsistent rains, total rainfall, heat stress, solar radiation, seasonal availability of forages, etc). No significant effect of parity/age of the cow on the weaning weight was detected, but age at weaning was found to be a very important determinant of the weaning weight. The earlier the calves were weaned, the lower their weaning weight, but the higher their adjusted weaning weight to an age of 205 days. The season at which the calves were weaned did not significantly affect their weaning weight, despite the apparent advantage that dry season born calves had over their rainy season born counterparts. A weaner productivity index was used in order to determine the best calving season. This index showed that cows calving in the dry season had significantly (P<0.001) higher indexes than those calving during the rainy season (113.1 ± 2.0 and 95.0 ± 1.5 kg, respectively). This index also differed with the sex of the calves (107.7 ± 1.6 and 100.4 ± 1.4 kg for male and female calves, respectively) and the month of birth. These results indicate that the best 3 months for calving span from November to January, with a corresponding mating season between February and April. Pre-weaning weights (1, 3 and 6 months of age) were significantly (P<0.001) influenced by the month, season, year of birth, and by the sex of the calf. The sex of the calf, month and year of birth but not season of birth significantly (P<0.05) affected average daily gain of Ngaoundere Gudali calves. The mean ADG were 1.1 ± 0.4, 0.8 ± 0.2, and 0.7 ± 0.1 kg/day from birth to 1, 3 and 6 months of age, respectively. Male calves consistently grew faster than their female counterparts at these 3 pre-weaning stages. Post-weaning weight and growth rate were significantly affected by the sex of the calf for the period considered (from 9 to 36 months of age), with bull calves retaining a higher growth rate. However, all calves experienced negative growth rates between the ages of 8 and 9 months (weaning took place at 8 months ± 2 weeks), due to either the stress of weaning or due to the new environment to which the weaners were moved. Season of birth no more significantly affected post-weaning weight and ADG from the age of 18 months. Interestingly, the study showed that the Ngaoundere Gudali heifers reach bodyweights of 222 and 281 kg at 24 and 30 months, respectively, meaning that an earlier age at first breeding could be contemplated. The average length of the oestrous cycle of the Ngaoundere Gudali cow was 21.8 ± 0.5 days, ranging from 15 to 35 days. Most oestrous cycles (67.9%) had a length comprised of between 18 and 22 days, while 18.5% lasted between 23 and 28 days. The oestrous cycles were significantly (P<0.05) longer during the dry season (24.1 ± 0.7 days), compared to the rainy season (20.6 ± 0.5 days). Historical data from the AI herd showed that out of 1504 services performed at natural oestrus during a 9- year observation period, only 28.9% of the oestrous manifestations were detected during the dry season (November to March) and early rainy season (April and May), compared to 71.1% from June to October. The seasonal peak occurrence of oestrus correlated well with the seasonal concentration of births in the herds to suggest that oestrous manifestation in the Ngaoundere Gudali cow is more readily expressed during the rainy season. The most consistent sign of oestrus was the acceptance of the female in oestrus to be mounted by a teaser bull or another female, for an oestrous period that averaged 9.8 ± 0.6 hours (range of 5 to 13 hours). However, it was suspected that in this study, due to the low duration and long intervals between observations, the onset and termination of some oestrous periods could have been missed. Thus the shorter duration of the oestrus recorded. Serum progesterone concentrations were at the lowest (0.4 ng/ml) between days 1 and 3 of the oestrus cycle, before rising from day 5 to reach peak values (5.5 to 6.5 ng/ml) between days 15 and 19. These serum progesterone levels declined rapidly thereafter to reach very low levels by the day of oestrus. Uterine involution was completed in Ngaoundere Gudali cows by 45 to 60 days after calving, earlier in a larger proportion of cows with calves under restricted suckling (RS) management, than in the unrestricted suckling (US) group. 26.3% of the cows in the RS group and 0% of the US group were in oestrus within 75 days postpartum. Between 75 and 90 days postpartum, 84.2 and 15.8% had been observed in oestrus and mated in the respective groups. The mean postpartum interval from calving to first oestrus was significantly (P<0.05) shorter in the RS (83.4 ± 5.1 days), compared to the US group (126.4 ± 5.1 days). The intercalving period was estimated at 397.9 ± 5.0 days, significantly (P<0.01) longer in cows that were suckled ad libitum (419.4 ± 6.5 days), than in cows in the RS group (376.4 ± 3.2 days). A high correlation was found between the reproductive tract score at 60 and 75 days postpartum (r = 0.6) and the conception rate of the cows. The overall mean weaning weight was 114.8 ± 2.7 kg for all the calves - significantly (P<0.05) higher for the US than for the RS regimes (134.1 vs. 95.3 kg, respectively). This indicates that restricted suckling had slowed calf growth and decreased weaning weight. Pre-weaning growth rate was significantly higher in the US than in the RS group. Calves that were in the US group and weaned at 8 months were 34.9, 28.7 and 71.1 kg heavier than the RS calves weaned at 8 months, US and RS calves weaned at 6 months, respectively. Postweaning growth rate to 12 months of age was significantly different (P<0.01) for the two weaning regimes and it was found that early weaned calves grew faster (176 g/day) than those weaned at 8 months (123.8 g/day) - revealing some compensatory growth from weaning up to a yearling age. The level of serum IgG in 35 newborn Gudali calves (72 hours following birth) recorded a mean concentration of 7.8 ± 0.5 gII (range of 0.04 to 12.7 gII), far below the levels quoted in the literature at 24 to 48 hours following birth. These values point either to a low concentration of IgG in the colostrum of Ngaoundere Gudali dams, leading to an insufficient intake by the calves, or to a limited absorption of the ingested colostrum. Overall, the present series of studies have contributed to the characterization of the most important aspects of the Ngaoundere Gudali reproductive and productive parameters. In general, the results obtained are indicative of low reproductive performances that seem to be linked to the seasonal availability of feed, which in turn depends on the seasonal and annual variations in rainfall. Certain management practices with a potential to improve the reproductive efficiency of the cows were identified (shorter breeding season, restricted suckling, early weaning, etc). An improvement in the reproductive efficiency of Ngaoundere Gudali cows would contribute to improve the overall productivity of the traditional cattle production systems in the Adamawa Highlands of Cameroon.