Mineral supplementation and reproductive rate of beef cows grazing tropical natural pastures in Malawi (part I)

Mineral supplementation and reproductive rate of beef cows grazing tropical natural pastures in Malawi (Part I)

*Bunda College of Agriculture, P.O. Box 219, Lilongwe, Malawi **University of Florida, Gainesville, FL 32611-0691 USA

An experiment that started in June 1984 was conducted at Kuti Ranch near Salima in the central region of Malawi, involving 161 Brahman crossbred cows divided into four groups of about 40 cows. The objective was to determine response of cows to mineral supplementation identified as deficient in a previous study. The animals grazed tropical natural pastures and were given mineral supplements throughout the year. The treatments were as follows: Group 1 -- salt + phosphorus, Group 2 -- salt + phosphorus + copper + cobalt + selenium, Group 3 -- salt only, Group 4 -- salt + copper + cobalt + selenium. Phosphorus was given as a mixture of monocalcium and dicalcium phosphate, copper as copper glass boluses (including cobalt and selenium) and salt as sodium chloride crystals. Cows were not supplemented the first breeding season. Supplementation of salt and phosphorus started the third breeding season in 1986. Thus, the experiment had complete treatments in 1986-87 breeding season. Samples were collected of bone and liver biopsy, blood, and forages according to species grazed by the cattle and the soil upon which the forage grew and analyzed for a number of minerals. There was no effect of phosphorus or copper on pregnancy diagnosis rate. Also, there was no effect of phosphorus on calving rate of the cows based on results for two years of mineral supplementation. Mineral supplementation did not increase the calving rate significantly from the average of 48% for the animals supplemented with only salt during the dry season. Mineral analysis indicated that phosphorus, copper and zinc in the animal tissues, forage and in soil were deficient. However, phosphorus supplementation did not raise the phosphorus concentrations in the bone above critical level, whereas copper supplementation raised copper twofold.

The low national calving rate of 55%-60% (1) even in well-managed herds with adequate grazing areas and adequate disease control (52% calving rate, a nine-year average for Dzalanyama 1973-1982 and a 10-year average of 48% for Kuti Ranch) has led to a suspicion of mineral deficiencies. Lack of mineral supplementation for grazing cattle has often been suspected as a major cause of low calving rate (Rudert and O'Donovan 1976; Ward 1968). Mtimuni (1982), in a national survey, identified phosphorus, copper and zinc, besides sodium, as the minerals that are deficient in soil, plant and animal tissues in Malawi. However, the true test of a mineral deficiency in animals still remains the production response resulting from mineral supplementation (McDowell et al 1983).

Mineral supplements which are sometimes available in the market in Malawi are formulated for South African conditions where mineral investigations were conducted extensively in the thirties and forties (Dutoit et al 1940; Theiler et al 1924). Mineral supplements have to be site specific to benefit the local farmers. Regional reconnaissance or a systematic mapping survey is a technique whereby mineral analysis from animal tissues (or fluids), forages and soil samples are used to identify mineral deficiencies. A further step in systematic mapping survey techniques is to test the biological response of mineral supplementation for animals. Testing mineral response in animals is very expensive. This is perhaps the reason many countries rely on mineral formulations based on conditions existing elsewhere or mineral formulations based on scanty mineral analysis of only soil or forage.

Salt (NaCl) and phosphorus supplementation have been reported to increase the calving percentage from as low as 25% to 75% (McDowell et al 1983). Ward (1968) fed bone meal and Rudert and O'Donovan (1976) fed monosodium phosphate to beef cows grazing natural pasture in Zimbabwe; but, they did not find any response to calving in three to six consecutive calving seasons. The phosphorus status of the soil upon which the forages grazed by the animal grew was not reported.

Phosphorus, copper and salt (NaCl) were found to be deficient in animal tissues, forage and in soil in many areas of Malawi (Mtimuni 1982). It was, therefore, the objective of this study to test calving response in Brahman cattle grazing natural pastures at Kuti Ranch, to minerals (sodium chloride, phosphorus and copper) which had been identified deficient in the previous survey. An additional objective was to establish the mineral status of cattle based on soil, forage and animal tissue levels over a three-year period.

The test for response in animals was done at Kuti Ranch. The ranch covers 2,693 ha and is located 14 latitude south and 34 longitude east at an elevation of 500 meters, with slightly undulating to flat terrain in the central region of Malawi. Mean maximum temperatures are from 30 to 35 degrees centigrade occurring between September and mid December. Mean minimum temperatures are from 15 to 20 degrees centigrade occurring between the end of April to early September. During the rest of the year, temperatures are from 20 to 25 degrees centigrade. Mean annual rainfall is about 1,150 mm, with considerable variation. The vegetation is savanna type with open grasslands. The soils are Eutric fluvisols, and vertisols also occur in a number of the paddocks (Lowole 1981).

The ranch keeps about 1,000 cattle of various classes, including about 400 breeding beef cows, and the overall estimated stocking at the ranch is about 4.9 ha per livestock unit (1 livestock unit = 340 kg). The objective of the ranch since 1976 has been to breed crossbred cows (Malawi Zebu-Africander and Sussex) with Brahman bulls (and no new stock of Africander and Sussex has been introduced since 1976) to obtain steers for use as work oxen. The breeding season starts on December 1 and ends March 31, and all open cows by July are bred from August 1 to September 30 every year. The main calving season starts in September and ends in November. The calves are weaned at 8 months when they are expected to weigh 90 kg.

The average calving rate at Kuti Ranch is about 48% (1976-1986), about 40% of the cows calve every two years, and about 60% of the cows calve longer than a two-year interval. The problem of low calving rate in Brahman cattle is not unique to Kuti Ranch only, but other livestock centres, for example Dzalanyama Ranch and Mikolongwe Livestock Centre, experience the same problem, indicating that low calving rate is more widespread in Malawi as indeed several reports indicate (Livestock and Meat Study 1983; Mtimuni 1982).

One hundred and sixty-four cows (Brahman, Malawi Zebu, Africander, Sussex) were randomly divided into four treatment groups of 42 animals each in December 1984 when the breeding season started. The final supplement treatments were as follows: Group 1 -- salt + phosphorus, Group 2 -- salt + phosphorus + copper, Group 3 --salt only, Group 4 --salt + copper. However, no minerals were received by any of the groups in 1984-1985. In 1985-1986, salt was given to all groups and phosphorus to groups 1 and 2. The experiment had the complete treatments in the 1986-1987 breeding season.

The phosphorus was given as a mixture of feed grade monocalcium and dicalcium phosphate, copper as copper glass boluses, and salt as sodium chloride crystals. Copper boluses also contained cobalt and selenium.

Salt (NaCl) and a mixture of feed grade monocalcium and dicalcium phosphate (21% phosphorus, 18% calcium, 0.21% fluorine) was supplemented at a rate of 80 g (50 g NaCl + 30 g monocalcium and dicalcium phosphate) starting in December 1985. The breeding season started on Jan. 23, 1986, and ended April 23 of the same year. Since some cows were pregnant, only four bulls were used. Pregnancy diagnosis was done in July 1986, and all open or non-pregnant cows were bred from Aug. 1 to Sept. 30.

Copper boluses (13.4% copper as CuO, 0.3% selenium as sodium selenite, and 0.5% cobalt as CoO₄ (Cosecure, Wellcome, Chance Pilkington Ltd., St. Asaph, N. Wales, U.K.) were administered to groups 2 and 4 by balling gun, two boluses per animal to last for one year according to the specifications of the manufacturer. The breeding season with only four bulls started again on Jan. 23 and ended on April 23, 1987. Pregnancy diagnosis was done in July, but open animals were not rebred because it was not easy to trace the number of cows exposed to the bull in a mixed herd. Cows started calving in October and continued until January 1986. The calves were identified and weighed. Unfortunately, lack of tags made it difficult to weigh calves immediately after they were born the first two years. The calves were weaned only if they weighed 90 kg at about 8 months of age.

Animals in groups 1 and 2 grazed together as did animals in groups 3 and 4. The animals were managed as the other animals on the ranch with minimum intervention, except experimental cows were weighed every month. The animals grazed six paddocks covering about 450 ha. Animals were bred in 1984 without mineral supplementation. Eight bulls on the station were randomly chosen and randomly assigned four to each of the combined groups. Breeding started Dec. 1, 1984, and ended March 31, 1985. Pregnancy diagnosis was done in July. As many animals in groups 1 and 2 were not pregnant, all the open cows were rebred from Aug. 1 to Sept. 30.

Bone and liver biopsy samples were collected from the same four cows on each treatment every December and every May since December 1984. Forage and soil samples were collected at the same time as blood samples. Blood samples were collected regularly in February, April, July and October each year from no less than 20 animals from each group every year since December 1984.

Predominant forage species grazed by the animals, for example Hyparrhenia species, Panicum maximum and Andropogon gayanus, were collected separately according to species in each paddock at the same location, three locations per paddock along a diagonal line since December 1984. Stainless steel scissors and knives, and disposable plastic gloves were used when collecting the samples to minimize mineral contamination. Soil samples were collected using a soil auger and disposable gloves.

Serum was obtained from the blood samples in the evening and stored, along with the biopsy samples in a refrigerator. Forage samples were first sun-dried then dried at 80 degrees centigrade in a drying oven, and soil samples were dried and extracted with Brays solution. The soil and the forages were analyzed for the same minerals -- Ca, P, Mg, Na, K, Fe, Zn and Cu. The serum samples were analyzed for Ca, P and Mg; bone biopsy samples were analyzed for Ca, P and Mg; and liver biopsy samples were analyzed for Fe, Zn and Cu according to the standard procedures in mineral analysis (Fick et al 1979). All the samples were analyzed on Perkin-Elmer Atomic Absorption 3030. Results were analyzed according to analysis of variance with Duncan's multiple range test (Duncan 1955).

Mineral supplementation did not greatly influence the calving rate nor the pregnancy diagnosis test of cows on experimentation (Table 1). The calving rate was within the 10-year average of 48% for Kuti Ranch where cows received mineral supplementation (salt and/or bone meal whenever available) during the dry season only. This is despite removal of nine cows from the experiment which did not calve in three calving seasons.

There was a drop in calving rate in all groups, except in Group 2 from 1985-1986 to 1986-1987 breeding season (Table 1), and the drop by 23.3% was greatest in Group 4. The calving rate in the third year was much lower than in the first year without mineral supplementation, except for Group 2, which increased by 46.2%, and the drop by 43% was greatest in Group 3. Calving rate was also much lower for the ranch in 1986-1987 than in the previous two years of the experiment.

The average calving rate for two years of mineral supplementation compared to the first year was higher in Group 2 and Group 4, but there was a drop in average calving rate after mineral supplementation in groups 1 and 3, and the drop by 36% was greater in Group 3. The percentage of cows with two calves in three years was higher in groups 1 and 2 and was lowest in Group 3. Although most cows in Group 3, which had two calves in two years, calved consecutively, cows in groups 1 and 2 were evenly divided between consecutive and alternate calving. The overall percentage of cows with two calves in three years was 32%, as opposed to 60% for the Kuti Ranch. These effects could not be tested because of lack of replication.

It was the intention of the experiment not to interfere with the general management of the ranch. Therefore, no special effort was made to acquire new and young bulls for the experiment although the need was more than apparent in the third breeding season. Consequently, the treatment effects have been confounded with bull effect in the third year, and an isolation of the effect can only be done by using young and fertile proven bulls. The only observed effect of mineral supplementation was that cows looked in a very good condition (although the condition was not scored), and only two or three animals in groups 1 and 2, as opposed to half in groups 3 and 4, required emergency survival measures for three weeks during 1987.

The most devastating economic effect of phosphorus deficiency is documented to be reproductive failure, with phosphorus supplementation drastically increasing fertility levels in grazing cattle in many parts of the world (Theiler et al 1924; McDowell et al 1983; McDowell 1985). In phosphorus deficient areas, cattle are reported to produce a calf every other year or longer and sometimes without even coming into estrus (McDowell et al 1983). These animals require longer calving intervals to build up phosphorus reserves.

Table 1: Pregnancy diagnosis (PD) and calving percentages of cows before and after mineral supplementation.

Group No.	1	2	3	4

	Without Supplementation*

No. of cows	40	39	37	36
PD, %**	40.0	33.3	78.4	44.4
No of calves	18	15	24	14
Calving %	45.0	38.5	64.9	38.9

	1985-1986*** With salt + P

No. of cows	40	39	37	36
PD, %	52.5	33.3	48.6	30.6
No. of calves	17	19	17	23
Calving %	42.5	48.7	45.9	63.9

	1986-1987**** With salt + P + Cu

No. of cows	31	32	27	31
PD, %	32.3	81.3	55.6	38.7
No. of calves	11	18	10	9
Calving %	35.5	56.3	37.0	29.0

Cows with 1 calf/3 yrs, %	37.5	61.5	56.3	50.0
Cows with 2 calves/3 yrs, %	35.0	33.3	27.0	30.6
Cows with 3 calves/3 yrs, %	2.5	0.0	5.4	0.0

* All groups received no mineral supplement.
** Pregnancy diagnosis test positive.
*** Salt (NaCl) given to all groups, phosphorus to groups 1 & 2.
**** Salt given to all groups, phosphorus to groups 1 & 2, copper (+ cobalt + selenium) to groups 2 & 4.

Phosphorus supplementation using bone meal increased fertility of cattle significantly and was recommended by Du Toit et al (1940) in South Africa. Bone meal did not increase calving percentage of grazing cattle in Zimbabwe over six consecutive calving seasons (Ward, 1968). Cohen (1975) did not find convincing evidence that phosphorus alone was responsible for the reproductive failure in cattle in Africa. Any increase in calving rate attributed to phosphorus in bone meal might be due to high protein content of the bone meal which supplied about 10% of the protein requirement (Cohen 1975). Phosphorus supplementation as monosodium phosphate did not increase the calving percentage of grazing cattle in Zimbabwe over three consecutive calving seasons (Rudert and O'Donovan 1976). Since phosphorus supplementation in this study was only 21% of the requirement (30 g P/day, NRC 1976) but a third of what Du Toit et al (1940) recommended by feeding bone meal and that the forages were deficient in phosphorus, failure to demonstrate any significant effect of phosphorus on calving rate may be attributed to calves sharing the already low quantities of monocalcium and dicalcium phosphate supplied to the cows. For Black et al (1943) had fed Brahman x Shorthorn and Brahman x Hereford heifers daily 6.5 g bone meal or disodium phosphate and they were able to increase calving rate. In addition, the low calving rate in the third breeding season of this experiment may be caused by the use of some old bulls.

One of the most documented effects of phosphorus is the increase in weight of the animals during the rainy season (Cohen 1975; Van Schalkyk and Lambard 1969). The phosphorus-supplemented animals lost less weight, and the calves looked healthier than animals supplemented with salt only or salt plus the three trace minerals. All calves were weaned at 8 months of age, weighing not less than 90 kg, and the weaning rate was 90%.

Serum phosphorus was above the critical level of 4.5 mg/100 ml in 1985. However, for 1986 and 1987, a high percentage of animals were below the critical level, particularly 1986 (Table 2). Animals receiving phosphorus in 1986 had higher (P < 0.05) serum levels of the mineral.

Bone is the tissue of greatest value in assessing phosphorus status of animals (Committee on Animal Nutrition 1973). Phosphorus was deficient in all animals in all groups in all three years (Table 2).

Supplementation of phosphorus at 6.3 g/head/day did not increase phosphorus above the critical level in the bone biopsy samples although there was a non-significant increase in bone phosphorus after supplementation (Table 2). Black et al (1943) fed 6.5 g bone meal/head/day equivalent (0.91 g P/head/day) and they were able to increase calving rate in beef cows. Ward (1968) fed even higher quantities of bone meal but was not able to increase the calving rate in six calving seasons.