6. Environmental influences on performance traits at Mkwaja

Effect of specific factors on performance
Development of effective management systems
Increasing selection efficiency

Environmental factors that influence traits contributing to overall productivity are important on two counts. First, knowledge of certain factors and the magnitude of their effects on performance may enable development of more effective management systems for increased beef production. Second, in genetic improvement programmes, adjusting data for known environmental effects allows valid comparisons among individuals to be made, thus increasing selection efficiency.

The environmental influences measured were year and season of calving or birth, cow age, calf sex, and area and location of herd. The amount and timing of trypanocidal drug treatments required and their interactions with environmental influences were evaluated simultaneously. The performance traits involved were calving interval, pre-weaning mortality, pre-weaning growth characters, and cow productivity (weight of weaner calf per cow per year).

The significance of environmental and drug treatment effects and their interactions on all performance traits are indicated in Table 14. The actual effects related to year of calving or birth, season of calving or birth, age of cow, sex of calf, and ranch area and location are presented in Table 15.

Effect of specific factors on performance

Year of calving or birth
Area and location on ranch
Cow age
Season of calving or birth
Sex of calf

Year of calving or birth

Year of calving or birth significantly affected all traits (P < 0.05 for calving interval, P < 0.01 for others). No general trends over years were apparent. Productivity was above average in 1981,1980 and 1976. In 1980, the policy on use of trypanocidal drugs was changed, so that animals thereafter received prophylaxis approximately one third more often, but this was confounded with any other year effects. 1976 was the last year of a 3-year drought, and cows calving then had the longest subsequent calving interval. However, low calf mortality and high weaning weights resulted in above-average productivity in that year. Significant interaction effects between year and season for all traits presumably reflected the very variable seasonal rainfall values shown in Table 5. Significant interaction effects between year and geographical area of ranch for all traits are discussed below.

Area and location on ranch

Area of ranch had a significant effect on calving interval and birthweight (P < 0.01) and cow productivity (P < 0.05). Performance was superior in all three traits in the northern area (Table 15).

In 1967 an extensive programme of bush clearing was started in the northern area, and by 1981 about 9600 ha had been cleared, approximately 50% of the area. No bush clearance was attempted in the southern area. From November 1977 to December 1978 a tsetse fly control project was carried out in the northern area, the objectives being to reduce the tsetse population by application of insecticide, then to release sterile male G. m. morsitans. A tsetse barrier 1 km wide around the northern area was also completed at this time. The southern area served as the untreated control. Over the 9 years the calving interval was 6% shorter, birthweights were 7% higher and cow productivity was 6% greater in the more developed northern area.

Table 14. Mean squares for performance traits of cows and calves, Mkwaja Ranch, 1973-82.

Source	df	Calving	Calf	Birth-	Pre-weaning	Weaning	Cow
		interval	mortality	weight	growth	weight	productivity
		(days x 10)	(% x 10)	(kg x 10)	(g/day x 10-2)	(kg)	(kg)
Year (A)	8	6505*	7225**	10127**	10389**	58915**	94952
Season (B)	3	2434**	3686**	3716**	656**	3307**	16113
Age (C)	3	9694**	606	1516**	5186**	33577**	31747
Sex of calf	1	2243**	1287	72040**	36466**	295301**	64976
Area (D)	1	4052**	1092	25599**	12	1764	12418*
Location/northern area	1	27	3686*	552	23678**	130720**	135312
Location/southern area	2	398	973	704*	404*	1539	2174
Number of Samorin treatments (E)	3	137	14956**	368	1426**	9285**	4228
Timing of first Samorin treatment (F)	3	302	2231*	1807**	266*	2171**	1728
Berenil treatment (G)	1	1073	3	198	407*	2737*	53
A x B	24	2239**	6600**	4289**	3998**	23037**	27002
A x D	8	9295**	3138**	9438**	1932**	12448**	52643
A x G	7	414	589	2148**	2132**	12290**	7508
B x D	3	836	299	747**	1334**	7191**	7584*
C x E	9	782*	1157	222	426**	2533**	2879
C x G	3	920*	3352**	652**	3463**	20255**	32512
D x E	3	241	771	398	99	404	3656
D x F	3	214	721	3224**	33	170	3422
D x G	1	993	436	5100**	619*	6752**	6704
E x G	3	319	1447	98	2376**	14219**	12419
F x G	3	291	829	682**	105	664	7746*
Residual		333	709	165	95	573	2099
Residual df		11905	18172	16181	16181	16181	11905

* = P < 0.05
** = P < 0.01

Table 15. Estimated least squares means for performance traits of cows and calves, Mkwaja Ranch, 1973-82.

		Calving interval		Calf mortality		(Numbera)	Birthweight	Calf growth	Weaning weight	Cow productivity
		(Numbera)	(Months)	(Numbera)	(%)		(kg)	(g/day)	(kg)	(Numbera)	(kg)
Overall		11999	16.58	18266	8.9	16275	25.5	446	132.3	11999	93.8
Year
	1973	1516	18.11a	1943	10.7bcd	1781	19.5d	492a	137.4a	1516	87.9a
	1974	1829	17.16a	2376	13.7d	2095	25.2a	436b	129.4b	1829	85.1a
	1975	1670	16.33b	2365	10.3bc	2133	25.2a	428b	127.7b	1670	89.5a
	1976	1550	18.44a	2387	5.9ab	2225	27.8b	471a	140.5ad	1550	105.1c
	1977	1172	16.97ab	1913	12.2cd	1627	27.4b	378c	117.8c	1172	76.2b
	1978	910	17.37a	1507	11.5cd	1369	25.5a	373c	114.8c	910	70.5b
	1979	842	17.42ab	1476	7.3abc	981	26.0ac	416b	125.6b	842	84.3ab
	1980	1503	14.30c	2040	5.1a	1928	26.5c	527d	152.7e	1503	121.1cd
	1981	1007	13.16d	2259	3.4a	2136	26.4c	493a	144.4d	1007	124.5d
Season
	January to February	874	17.60c	1354	12.1b	1143	26.5a	432a	129.9a	874	87.1a
	March to May	1709	16.65a	2428	8.4a	2173	24.6b	445b	131.0a	1709	90.9a
	June to September	7872	15.76b	11710	6.5a	10521	25.0c	453b	133.4b	7872	100.0b
	October to December	1544	16.32ab	2774	8.6a	2438	26.0d	454b	134.6b	1544	97.2b
Age
	3 to 4 years	3591	17.96c	4854	8.9	4351	25.4a	448a	132.6a	3591	86.0b
	5 to 6 years	3819	16.52a	5690	8.7	5009	25.7b	462b	136.3b	3819	97.8a
	7 to 8 years	2777	15.77b	4303	9.6	3820	25.8b	451a	133.6a	2777	97.8a
	9 years and over	1812	16.08ab	3419	8.5	3095	25.2a	423c	126.5	1812	93.6c
Sex of calf
	Male	5912	16.72a	9108	9.2	8067	26.2a	461a	136.5a	5912	96.1a
	Female	6087	16.45b	9158	8.6	8208	24.8b	431b	128.0b	6087	91.5b
Area
	Northern	5365	16.09a	8088	9.4	7223	26.4a	446	133.0	5365	96.5a
	Southern	6634	17.08b	10178	8.4	9052	24.7b	447	131.5	6634	91.0b
Location/area
	North 1	4130	16.16	5816	7.5a	5237	26.1	497a	145.0a	4130	113.5a
	North 2	1235	16.01	2272	11.3b	1986	26.6	394b	120.9b	1235	79.6b
	South3	2847	17.06	4483	8.4	3955	24.8ab	444ab	131.1	2847	91.2
	South 4	2812	17.31	4149	7.5	3766	25.0a	440a	130.2	2812	89.4
	South 6	975	16.87	1546	9.2	1331	24.3b	456b	133.4	975	92.6

^a Number of observations for which records were analysed.
Within variable groups, row means followed by the same letter do not differ significantly (P<0.05).
If no letter is used, this indicates that the variable group did not show a significant difference in the analysis of variance.

There were significant interaction effects between area and year for all performance traits, and the interaction constants are shown in Table 16. Significant interactions were due mainly to relatively superior performance in the northern area in 1973 and 1978 and relatively inferior performance in 1976 and 1980. Biological events that could have a possible bearing on these results are not known for 1973: 1976 was the final year of a 3 year drought spell affecting both areas; 1978 saw the temporary reduction of tsetse population in the northern area: and 1980 saw the start of more frequent prophylactic treatments in both areas.

Table 16. Estimated least squares constants for interaction between area on ranch and year of calving or birth for performance traits, Mkwaja Ranch, 1973-82

	Estimated km squares constants for interaction between northern areaa of ranch and year
	Cow	Calving	Calf	Birth-	Calf	Weaning
	productivity	interval	mortality	weight	growth	weight
	(kg)	(days)	(%)	(kg)	(g/day)	(kg)
1973	14.9	-62	-2.5	1.6	-15	- 2.0
1974	1.3	-3	0.4	0.4	-1	0.1
1975	-1.6	9	0.6	-1.1	0	1.1
1976	-9.4	28	0.1	-1.3	-20	-6.1
1977	-0.5	- 15	3.0	- 0.2	12	2.6
1978	7.2	-19	-1.0	0	17	3.9
1979	1.7	-6	-1.4	0.1	7	0.7
1980	-9.3	54	-0.8	0.7	-7	-1.0
1981	-4.3	15	1.6	- 0.2	8	1.7

^a Estimated least squares constants for interaction between southern area and year are the same values with the sign reversed.

Significant interaction effects between area and season for calf weights and cow productivity presumably reflected different seasonal rainfall in the two areas.

Within the northern area, animals in location N1, where the bulk of the bush clearance had been carried out, had lower mortality and higher pre-weaning growth, weaning weight and cow productivity than those in location N2 There were only minor differences in birthweight and pre-weaning growth between animals in the three locations within the southern area.

Cow age

Cow age had a significant effect (P < 0.01) on all traits except calf mortality Calving intervals fell from 1796 months for 3 to 4-year-old cows to 15.77 months for 7 to 8-year-olds and increased slightly to 1608 months for cows aged 9 years and over Calf birthweight, pre-weaning growth and weaning weight were highest in progeny of 5- to 6-year-old cows then fell off gradually. Overall cow productivity was lowest in 3- to 4-year-olds, highest in 5- to 8-years-olds and fell off in cows aged 9 years and over.

For young cows, the longer calving interval, lower calf weights and lower cow productivity may reflect a higher nutritive requirement because they still had requirements for growth in addition to those for lactation and maintenance. The lower trait values for the older cows is interpreted as being the result of reduced ability to cope with nutritional and other stress factors associated with the aging process.

Season of calving or birth

Season of calving or birth significantly affected all traits (P < 0.01) When seasonal effects on each performance trait were expressed as a percentage above or below the overall mean (Table 17), the largest seasonal differences were in calving interval, calf mortality, and cow productivity. Calving in the period June to September, the second dry season, gave the best results for these three traits.

Calving intervals following parturition in June to September were 4.9% shorter than the overall average. Calving during this period allowed conception during the short rains Mortality rates following birth in June to September were 27% tower than the overall average The highest mortality occurred in calves born in January to February: these animals had the long rains to contend with before weaning Cow productivity following calving in June to September was 6.6% higher than the overall average

Seasonal effects on birthweight, pre-weaning growth and weaning weight were less important While calves born in January to February had the highest birthweights, their tow pre-weaning growth resulted in their bring the group with the lowest weaning weights

Table 17. Effect of season of calving or birth on performance traits, Mkwaja Ranch 1973-82.

Season	Percentage above or below overall mean
	Calving intervala	Calf mortalitya	Birthweight	Pre-weaning growth	Weaning weight	Cow productivity
Jan -Feb.	6.2	36.0	3.5	-3.1	-1.8	-7.1
March - May	0.4	-5.6	-3.5	-0.2	-0.9	-3.1
June - Sept.	-4.9	-27.0	-2.0	1.6	0.9	6.6
Oct. -Dec.	-1.6	-3.4	2.0	1.8	1.8	3.6

^a Negative value more desirable

Sex of calf

Sex of calf had a significant effect (P < 0.01) on all traits except calf mortality. Cows suckling a male took 8 days longer to conceive than those suckling a female Male calves were 5.6% heavier at birth, grew 7% faster and were 6.6% heavier at weaning Cows producing male calves had an average productivity 6% higher than those with female calves.

Development of effective management systems

Evaluation of environmental influences on performance has indicated a number of important points in relation to management and the development of more effective management techniques.

Superior animal performance in the northern area where bush clearance had been undertaken suggests that an evaluation of the costs of this work in the two northern locations, relative to the value of the increased production, should be attempted. This would allow logical decisions on the possibilities of further bush clearance and pasture improvement in other locations The significant area by year interactions showed relatively higher productivity in the northern area in 1978 when a temporary reduction in the tsetse fly population had taken place. This suggests that further evaluation of approaches such as the use of traps to reduce the tsetse population could be relevant.

From the analysis of age of cow effects, the fact that rows did not decrease in overall productivity until aged 9 yeas and over (consistent with numerous reports in the literature from non-trypanosomiasis risk situations) indicates that the multiple in mutation of trypanocidal drugs (approximately 40 times by 9 years of age) did not raise any serious problems for animals

The calving season was shown to have a major effect on productivity Calving in the optimal season resulted in a productivity 9% higher than that over the rest of the year The fact that two thirds of cows were already bred to calve in this season shows this effect to have been well recognised. However, further emphasis on the optimal calving season would raise productivity still mom

Increasing selection efficiency

The major effects of year, season, cow age and calf sex on virtually all performance traits, and of location and area of ranch on several traits, illustrate the importance of taking these environmental influences into consideration in analyses Highly significant year and season of calving or birth effects and their interaction were as expected, in view of the very marked differences between yeas and seasons in terms of rainfall and pasture availability Cow age effects were consistent with numerous reports id the literature Very young cows that still have a nutritive requirement for growth show longer post-partum intervals and provide a poorer maternal environment for their calves than do older cows Very old cows also have lower production capabilities as a result of their reduced ability to cope with nutritional and other stress factors associated with the aging process Finally, the calf sex differences were entirety consistent with reports in the literature for suckling calf situations