Journal of Animal & Plant Sciences, 2009. Vol. 4, Issue 1: 267 - 275. Publication date 15 August 2009, http://www.biosciences.elewa.org/JAPS; ISSN 2071 - 7024

and increased number of replacement heifers In an attempt to combat this situation, needed for non-voluntary reproductive culls. imported Bos taurus germplasm from temperate In tropical countries, Bos indicus cattle countries has been widely used in crossbreeding are the predominant species kept by the with Bos indicus cattle in many parts of the majority of small holder farmers. These cattle tropics in order to improve cattle productivity are well adapted to tropical environments. They (Syrstad, 1985). In producing crossbred stock, possess a high degree of heat tolerance; they are either exotic bulls or semen from exotic sires fairly resistant to ticks and tick-borne diseases has been used on indigenous cows. It is of and to many other diseases, and have low interest to know which of the two methods of nutritional requirements (Cunnigham & Syrstad, mating is efficient under tropical conditions. 1987). However, these cattle are characterised The objective of this study was to investigate by high age at first calving, long calving intervals the factors affecting reproductive performance and low potential in terms of growth traits of Boran cows and heifers bred naturally with (Rege et al., 2001). Boran bulls and those artificially inseminated with Friesian semen under ranch conditions.

3 MATERIALS AND METHODS

conception. All animals underwent routine disease Data used in this study were collected from Kikulula control measures, especially against tick- borne Heifer Breeding Unit (KHBU) in Kagera region, diseases and worm infestations. North-western Tanzania. This region lies just below 3.2 Data classification: Seasons of birth or the equator between latitudes 1º 00´ and 2º 45´ calving were categorized into four classes as heavy south and is west of Lake Victoria. 3.1 Management of animals: New born rain season (March-May) =1; light rain season (September-December) = 2; early dry season calves were left to suckle until weaning at about 4 (January-February) = 3; and late dry season (Junemonths of age. Weaned and adult cattle grazed on August) = 4. Parturition numbers were coded 0 natural pastures, comprising mainly of Cynodon spp, (heifers) to 6. Types of mating were coded as NS Panicum spp, Eragrostis spp, and Andropogon spp for and AI for natural mating and artificial insemination, about 8 hours per day and were returned to the respectively. Data available for age at first calving sleeping paddocks in the afternoon. The length of (AFC) were from 1990-1996, records for calving grazing period depended on season, and it was interval (CI) and days open (DO) were available always longer during the dry season than during the from 1993-1998 and for number of services per wet season. Animals were occasionally

conception data were available from 1993-1999 and supplemented with rocks made from natural salt 1994-1999 for artificial insemination and natural (NaCl). Boran heifers and cows were kept into two mating, respectively. groups; one comprised of those intended for natural 3.3 Data analyses: Data were analyzed using mating by Boran bulls and the other group GLM procedures of SAS (2000). Model I was used comprised of those intended to be inseminated with to analyze age at first calving (AFC) in which year-Friesian semen to produce F1 crosses. Heifers joined season of birth and type of mating were included as the mating groups when they were about 15-18 fixed effects. Model II was employed in the analyses months of age. Detection of heat on cows and of calving intervals (CI) and days open (DO) whereby heifers to be bred by artificial insemination using fixed effects of parity, year-season of calving, and type Friesian semen was initially done using vasectomised of mating were fitted. In analysing the number of bulls or steers and later by close observation of the services per conception, fixed effects of year-season heifers and cows early in the morning and late of mating, , type of mating and parity were fitted in evening by trained staff and herdsmen. Heifers and the model (Model III). cows intended to be mated by Boran bulls were also monitored daily by trained staff and herdsmen to detect heat. Dates of mating were recorded daily and pregnancy diagnoses were done routinely in order to determine number of services that led to

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Journal of Animal & Plant Sciences, 2009. Vol. 4, Issue 1: 267 - 275. Publication date 15 August 2009, http://www.biosciences.elewa.org/JAPS; ISSN 2071 - 7024

D i The following models were used: = Effect of year-season of calving Model I: Y ijk = μ + D i + G j + (DG) ij + e ijk (1992…1998; 1…4;) G k F l Where;

Yijk = Age at first calving versus artificial) μ = Overall mean

(DF) il ; (GF) jk = Two- way interaction effects as

Di = Effect due to year-season of birth

per notations

(1990…1996; 1…4) = Random residual effect N(0, V ^{e 2} e ijkl )

Gj = Effect due to type of mating (natural versus artificial) Model III: Y ijkl = μ +D i + G j + F k +e ^ijkl (DG)ij = Two -way interactions effects as indicated by notations

Where;

= Random residual effect N(0, Ve2) eijk Y ^ijkl

μ Model II: Y ijkl = μ +D i + G j +F k + (DF) ij +

D ⁱ (GF) jk +e ijkl (1993…1999; 1…4;) G j Where; F ^k Y ijkl

= Calving interval or days open versus artificial)

μ = Overall mean = Random residual effect N(0, V ^{e 2} e ijkl ).

4 RESULTS 4.1 Age at first calving (AFC): The overall variation of 14.9 and 35.4 %, respectively (Table 3). mean AFC was 42.2 ± 0.23 months with a All effects (except parity and type of mating for coefficient of variation (CV) of 17.5 % (Table 1). DO) fitted in the model in analyses of CI and DO There were highly significant (P < 0.001) influences were highly significant (P < 0.001). However there of year-season of birth, and type of mating on AFC were also significant influences of parity (P<0.01) (Table 2). The interaction between type of mating and type of mating (P<0.05) on DO (Table 4). and year-season of birth was also significant (P < There was a decrease in both traits with increase in parturition number up to 4 ^th parity and then 0.05). There was no clear trend in AFC due to yearincreased considerably in subsequent parities. There season effect. However, there was tendency for was no specific trend in CI and DO with years. heifers born in late dry (June to August) season to However, the shortest CI was observed in the light calve earlier than those born during the heavy rain rain season while the longest CI was observed in the season months (March to May). Heifers bred by heavy rain seasons. Naturally bred cows tended to artificial insemination were older at calving than calve again about 50 days earlier than artificially bred naturally bred heifers by 3.9 months. 4.2 Calving intervals (CI) and days open cows. Open days for naturally bred cows were lower (DO): The overall means for CI and DO were 463.6 by 22.5 % compared to artificially bred cows. ± 1.93 and 183.4 ± 1.93 days with coefficients of

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Journal of Animal & Plant Sciences, 2009. Vol. 4, Issue 1: 267 - 275. Publication date 15 August 2009, http://www.biosciences.elewa.org/JAPS; ISSN 2071 - 7024

Table 1: Least squares means (standard error) for age at first calving (in months).

Overall 1458 42.2 (0.23)

***

Year-Season of birth

*** = P<.001

Table 2: Analysis of variance for age at first caving (AFC).

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