Factors Affecting High Mortality Rates of Dairy Replacement Calves and Heifers in the Tropics and Strategies for Their Reduction

The tropics is not an ideal location for calf rearing as the high temperatures and humidities introduce many potential disease problems to milk fed calves. In addition, the type of dairy farming (generally poorly resourced small holder farming) and the general lack of awareness of the long term implications of poorly reared stock do not encourage farmers to pay close attention to their calf and heifer rearing systems. Surveys of calf rearing systems in Asia, tropical Africa and South America highlight the high calf and heifer mortalities. A range of 15 to 25% pre-weaning calf mortality is typical on many tropical dairy farms. It is often as high as 50%, indicating very poor calf management. This contrasts with US findings of less than 8% mortality from birth to 6 months while surveys of Australian farmers report only 3% losses. Simple extension programs on farms in Sri Lanka and Kenya have drastically reduced calf mortalities and improved pre-weaning growth rates. Improved management strategies leading to lower calving intervals, higher calving rates, reduced still born and pre-weaned calf mortalities and fewer non pregnant heifers can supply many more dairy herd replacements than currently occurs. Such strategies can increase the number of replacement heifer calves in the herd from 15 to over 35%, thus allowing farmers to increase their herd sizes through natural increases. Simple management procedures such as ensuring adequate intake of good quality colostrum within the first 12 hours of life, housing and good hygiene to minimise disease transfer, providing clean drinking water, developing appropriate feeding protocols to encourage early rumen development and paying closer attention to climate control and animal health can all lead to improved calf vigour and performance. Good record keeping is also important so farmers can more easily identify susceptible calves and quickly treat potential problems. (


INTRODUCTION
The tropics is not an ideal place to rear young dairy stock.The harsh tropical climate introduces many animal health issues to dairy stock, particularly young calves (Aiumlamai, 1999).Furthermore in many countries, the type of dairy farming, generally poorly resourced small holder production systems (Moran, 2009), and the lack of awareness of the long term implications of poorly reared young stock (Moran and McLean, 2001), does not encourage farmers to pay close attention to their calf rearing systems.
This review documents the published data on current mortality rates of calves and heifers on tropical dairy farms, summarises the major reasons behind these high levels and reports on the success of farmer extension programs designed to reduce such mortalities to more acceptable levels.A series of Key Performance Indicators (KPI) are presented to provide quantitative guidelines to more acceptable levels of young stock management.
With increasing numbers of temperate, hence poorly adapted, dairy stock being imported into tropical countries, there is an urgent need for well planned and conducted farmer extension programs into young stock management (Moran, 2005).The progeny of such high grade Friesian cows are more susceptible to environmental and managerial stresses of tropical small holder dairy farming than local, more adapted calves (Vaccaro, 1990).Certainly, their high investment costs justify greater attention than they currently receive.
Considering the above, it is not surprising that the performance of young stock on most tropical and subtropical dairy farms is below that observed on temperate farms.Not only is the performance of young stock suboptimal, but such documentation is often difficult to find, particularly from Asia.This is evident in Table 1 in which most of the reported studies originate from Africa and South America.

PUBLISHED SURVEYS OF CALF AND HEIFER MORTALITIES IN THE TROPICS
The results of recent published surveys on mortality during calf rearing undertaken in tropical dairying areas are presented in Table 1.Altogether there are 17 tropical studies reported, many from Africa and South America, but several from Asia.The surveys covered a wide range of dairy production systems, from large scale grazing and feedlots to small holder systems.The highest pre-weaning mortality reported was 81%, from a survey of various Indian dairy systems (Tiwari et al., 2007) in which it was found that over half the 90 farmers ceased rearing any calves because they considered it uneconomic and it was cheaper for them to purchase all their herd replacements.This was an extreme  (2004) finding as the calf mortalities from the other reported studies varied from 10% to 35%, with one Vietnamese study reporting only 7% calf mortality.
Unfortunately, the studies documented a wide variety of ages over which calf mortalities were reported, making it difficult to fully interpret the data.However, a range of 15 to 25% pre-weaning and early post-weaning mortality rates would be typical on many tropical dairy farms.De Jong (1996) concluded that typical calf mortalities in the tropics, of 20 to 45%, are much higher than the 7 to 16% found in temperate regions because of high disease incidences and low feeding levels.This was further compounded by the low value of calves reared for meat or as replacement heifers.With regard to its potential economic impact, Amuamutu et al. (2006) calculated that a calf mortality rate of 20% can reduce dairy farm net profits by 38% compared with a target mortality level of 5%.
For comparison, the US Dairy Calf and Heifer Association (2009) present "gold standards" for dairy farms in the US, namely less than 8% mortality from birth to 6 months.Accepted norms are even lower in Australia (Moran, 2002), where a recent survey of 300 dairy farmers reported mortalities due to illness in calves reared to weaning to be only 3.3% (McNeil, 2009).
In his survey of 112 small holder farms in Ethiopia (Study 11 on Table 1), Jemberu (2004) reported that many of the farmers considered high calf mortality to be their number one herd health problem, compared to mastitis and infertility.On the 3 larger farms he surveyed, calf health problems were generally rated second behind mastitis.The incidence of calf health problems (that is the morbidity, in contrast to mortality) was higher on the 3 large farms, probably due to the greater number of calves being reared hence the potential for transfer of pathogens.However, despite the lower morbidity on small holder farms, mortality rates were higher, which the author interpreted as poorer management of sick calves and lack of easy access to health professionals for calf treatment.

Poor adaptation of imported stock
High calf mortality can also be the result of poor adaptation of exotic stock, for example when unacclimatised temperate or crossbred dairy stock are imported to the tropics, thus adding severe climatic stress to the other hazards of calf life.Vaccaro (1990) reviewed published data on the losses of European dairy breeds in the tropics, compared with European×zebu crossbreds.He separated data for European breeds as to whether the dams of the calves were imported or they originated from local or mixed origin.Approximately 15% of the imported calves failed to reach first calving or survive their first year of life.The published losses for different stages of young stock management for imported stock versus those of local/mixed origin were respectively: • 12.1 vs. 6.7% for abortions • 9.2 vs. 6.0%for stillbirths • 37.6 vs. 19.9%for calf deaths • 1.4 vs. 1.9% for calf culls Following weaning, the average wastage rates up to first calving for both groups amounted to 3.9% for heifer deaths and 6.6% for heifer culls.Over their herd life, imported stock only produced 2.6 calves compared with 3.1 for the local cows.Of every 100 females conceived, an average of 56 would have died or been culled before first calving in the case of imported dams, compared to only 39 for local dams.
Clearly, young stock management of exotic dairy stock or their progeny require closer attention than was the case prior to 1990, and possibly even after.Anecdotal stories during the 2000s of high mortalities among imported European dairy heifers, milking cows and their progeny calves and heifers in various Asian countries clearly indicate that their survival, as well as their post calving performance, could still be vastly improved.

The effect of grazing system on calf and heifer mortalities
Table 2 presents the results of a detailed survey, undertaken between 1996 to 1998, in which Bede (2008) and Bebe et al. (2003) documented the dairy herd dynamics of 1755 small holder farms in the highlands of Kenya.Of these farms, 987 had dairying enterprises as part of their mixed farming systems, and these had evolved to diversify the risks from dependence on a single crop or livestock enterprise.There was a large movement of weaned heifers from their home farms to other dairy farms as purchased heifer replacements.This strategy was used by farmers to reduce spending scarce feed resources on raising as yet, unproductive stock.
In the densely populated highlands of Kenya, farmers have had to intensify their dairy systems, by changing from free grazing to zero grazing, due to decreasing farm sizes.This led to changes in herd structure with more emphasis on milk production and increased stocking rates.High stocking rates were maintained through cut and carry feeding of Napier grass and crop residues, as well as sourcing additional fodder from neighbours and communal areas, together with purchasing more concentrates.
Farm sizes and total herd numbers decreased as farms became more intensive.In addition, calves were more likely to be bucket fed, they were fed less milk to 3 months of age, and were weaned earlier.The cows produced more milk.However, the proportion of heifers to cows was reduced because they were younger when sold to other farmers as heifer replacements.To provide sufficient heifers for a more intensive dairy industry, Bede (2008) recommended an increasing need for heifer breeding and growing farms.
Mortality rates of all classes of stock were very high, varying from 7 to 19%, but were not statistically significant across grazing systems for any of the animal classes.Losses due to animal diseases and their interaction with nutritional level were a major constraint to small holder dairying irrespective of grazing system.With 25% of the heifers dying and 22% being sold before breeding, only 53% of the heifers on the zero grazed farms could have become pregnant, produced milk hence generated farm income.
The high mortality and low calving rates then led to very high reproductive wastages, irrespective of the level of farm intensification.Together with the numbers of heifers sold, many of these farms were not able to maintain sufficient heifers for their own herd replacements.Herd sizes could not then increase without an external supply of replacement animals.Constraints to rearing replacement heifers on these farms ranged from high losses due to disease and malnutrition through to inadequate breeding services and credit.

REASONS FOR POORLY REARED CALVES IN THE TROPICS
Young stock require greater attention on tropical than temperate farms for a wide variety of reasons.Many of these (documented by Aiumlamai, 1999;Moran, 2005;2009;2011) are listed below.
• The tropical environment encourages the proliferation of many disease organisms that can reduce calf and heifer performance.These include some diseases that only occur in the tropics.• Dairy cows are essentially temperate animals which are most comfortable 6 to 18°C.The high tropical temperatures and humidities introduce specific climatic stresses that adversely affect calf and heifer feed intakes, growth rates and fertility.• As ambient temperatures approach body temperatures, stock rely more on cutaneous evaporation to remove residual body heat, and this is less efficient than other forms of heat loss, requiring increasing amounts of water and minerals.
• The stock on many small holdings are multi-purpose, being farmed for manure, meat and even draught, in addition to milk.Young stock management is often not a high priority on such farms.• Many of the dairy farms are small holdings where farmers often lack the resources to develop the most effective rearing systems for young stock, since most of their attention is directed towards income generation, namely through feeding and managing their milking cows.• The best way to reduce calf mortality is to practice good husbandry.Some common ailments can be treated by the farmer, but once a calf is seriously ill or large numbers are ill, expert veterinary advice should be sought and are not readily available.• Levels of shed hygiene are often suboptimal and this can have detrimental effects on monogastric animals such as milk-fed calves.• The nutritive value of tropical forages is generally poorer than that in temperate areas, so weaned heifers cannot grow as fast as they do in temperate regions without continual access to high energy and protein

EXTENSION PROGRAMS ON CALF AND HEIFER REARING IN THE TROPICS
Clearly there is much to improve on most tropical dairy farms (large scale or small holder) with regards to calf rearing.Several studies have quantified the impacts of simple extension programs on the improved performance of milk fed calves and weaned heifers in the tropics.

Sri Lankan study
To reduce calf mortality and age at first calving of heifers in an artificial insemination heifer calf rearing scheme during the 1990s, the Sri Lankan Ministry of Livestock Development instigated a heifer rearing program which supported smallholders with subsidised calf meal together with free mineral mixtures, drugs and acaricides for 30 months.Nettisinghe et al. ( 2004) reported the outcome amongst 100 smallholder farmers in the Coconut Triangle grazing and the peri-urban dairy production systems in the wet lowlands of Western Province.About 50 scheme and 50 non-scheme farmers were surveyed in the two production systems three years after the scheme finished and the data are summarised on Table 3. Virtually all the coconut grazers were part-time dairy farmers and supplied the formal milk market (receiving 10 to 11 Rs/L in 1999) whereas the peri-urban farmers supplied the informal milk market (receiving 19 to 20 Rs/L) while about one third of them were full time dairy farmers.About half the coconut grazing farmers stall fed their stock whereas most of the peri-urban farmers employed grazing only.Live weight at 24 months varied between breeds with Friesians being the heaviest (244-251 kg), followed by Australian Milking Zebus (AMZ, 219-241 kg), then Jersey (220-248 kg) with the lightest being Sahiwal (191-204 kg).Scheme heifers calved five months earlier than non-scheme heifers (32 vs. 37 mth) while coconut grazing herds calved three months earlier than peri-urban herds (33 vs. 36 mth).Across breeds, scheme Jersey and AMZ heifers were the youngest to calve (28 mth) with non-scheme Sahiwal being the oldest (42 mth).
Calf mortalities were lower in the coconut grazing compared to the peri-urban herds (18 vs. 21%) and on former scheme compared to the non-scheme farms (13 vs. 26%), with 50 to 60% of these mortalities occurring in the first three months of age.In coconut grazing herds, scheme farmers had more young female stock present per cow whereas there was no difference with the peri-urban farmers.
Most non-scheme and former scheme farmers applied deworming drugs and fed concentrates, whereas less than half applied deticking, while very few fed minerals.On the coconut grazing farms, former scheme farmers dewormed their calves more frequently and spent more money on concentrate feeding than did non-scheme farmers.There was no difference in these calf management practices in former scheme or non-scheme peri-urban farmers.The former scheme peri-urban farmers had higher costs and, because of their higher unit milk returns, they still achieved higher profits than did the coconut grazing farmers.
Poorer calf housing and low level of health management led to peri urban farmers' herds experiencing higher calf mortalities (see Table 3 for details).The coconut grazing farmers' herds produced more female stock per cow because of reduced calf mortality and younger ages at first calving.As the peri-urban farmers were less motivated to continue the improved calf rearing practices after the scheme was stopped, differences between former scheme and non-scheme farmers was much smaller.
The total production costs per in-calf heifer was considerably cheaper than the production of such animals, either by multiplication on state farms or imports.The authors calculated the cost of importing one exotic heifer to be the same as producing 16 well adapted heifers from this study.As in other countries, importation of exotic stock or multiplication of breeding stock on state farms has a poor record for expanding dairy populations.This study showed that only in certain production systems, in this case grazing under coconuts, were farmers likely to continue their improved heifer management practices after such government-supported schemes had finished.Lanyasunya et al. (2006) reported on a study of 120 small holder farmers in Kenya.It involved 60 test farmers, each provided with a chest girth tape (to estimate live weight), a spring balance and 10 L bucket (to weigh feed), a graduated jug (to measure milk) and a note book.An additional 60 control farmers continued with their traditional calf rearing practices.Each of the test farmers was also provided with moveable calf pens, compartmentalised with feed and water troughs, a daily heifer feeding schedule and forage planting material (which included improved tropical grasses and legumes).Test farmers regularly dewormed their calves and were visited every fortnight by extension staff.The dairy production systems were categorised as zero grazing, semi zero grazing or free grazing.

Kenya study
Pre-weaning growth rates were higher on the test farms (0.37 vs. 0.31 kg/calf/d) while calves offered legume forages (lucerne, Desmodium and Leucaena) grew faster than those only offered sweet potato vines (0.40 vs. 0.34 kg/calf/d).Mortality of heifer calves on zero grazed farms was lower than on semi zero and free grazed farms (6 vs. 15 vs. 20%).Mortality of bull calves was higher than for heifer calves, namely 13 vs. 6% on test zero grazed farms and 11 vs. 9% on control zero grazed farms.Heifer live weight at 24 months was 295 kg for zero grazed, 296 kg for semi zero grazed compared to only 240 kg for free grazed stock.
The key messages from this study included: • The beneficial impact of legumes enabled calves to more efficiently utilise low quality forages • Calf survival was improved because individual calf pens reduced disease transfer • Better performance of zero grazed stock was due to greater attention and improved management Tanzania study Lyimo et al. (2004) surveyed calf rearing practices on 60 small holder mixed farmers in Tanzania, with average herd sizes of only 3 cows.The area had fertile soils and a favourable climate, therefore cropping was given greater emphasis than milk production.Most farmers practiced restricted suckling.Calf survival and growth rates were low, but no mortality data were provided.Calves only weighed 96 kg at 6 months of age.Calves stayed with their dams until 4 to 7 days, were only provided with concentrates after 4 weeks and were weaned at 4 to 6 months of age.Calves were then stall fed with limited forages and poorly formulated concentrates.The major constraints to forage supplies were season of year, labour, ease of harvesting and distance to forage supplies.Farmers ranked the problems in calf rearing (mean score out of 8.0) as follows: • Labour (7.5) • Poor calf growth (7.0)• Diseases (6.5) • Little milk available (6.5) • Inadequate knowledge (4.8) • Mortality (3.4)They ranked the options for improvement (mean score out of 8.0) as follows: • Strategic feeding (7.7) • Provide more supervision and adequate management (6.3)• Use cheaper concentrates (5.5) • Farmer's training (5.5) • Adopt more appropriate housing (5.3) • Allocate more funds to calf rearing (5.0) • Accept technical advice (3.8)Many of their problems were nutritional with strategic feeding of milk, concentrates and forages seen as most important.These can be solved by improved farmer training and supervision to practice more appropriate husbandry.

Philippines study
A study was undertaken in the Philippines in which heifers were reared in different systems from 3 to 12 months of age and their performance was monitored (Payne, 1967).They were raised indoors or at pasture or a combination of the two, with or without strategic drenching for internal parasites (Table 4).
The indoor/outdoor system was used as it provided protection from heat stress during the day while at night, the larvae of most internal parasites would move down the stems of the pasture so the calves could safely select the better quality components of the forages.Calf growth rates were highest in the two drenched groups with access to grazed pasture while mortalities were lowest in the indoor groups.Payne concluded that even allowing for their higher mortality, the overall production was superior in the indoor/outdoor and drenched group.The most dramatic finding was the very high mortality in the outdoor and nondrenched group.

The importance of good colostrum feeding management
Although this has not been well documented in tropical calf rearing extension programs, temperate studies have shown good colostrum feeding management to be one of the keys to low calf mortality (Moran, 2002).In his survey of calf mortalities on small holder farms in Ethiopia, Jemberu (2004) reported a close relationship between calf mortality and hours post birth when calves drink their first colostrums.

KEY PERFORMANCE INDICATORS FOR REARING REPLACEMENT HEIFERS
There are many hidden costs arising from poor management of the replacement dairy herd.The milking potential of small stunted animals that do not calve until three years of age can be markedly reduced (Moran and McLean, 2001), while very high mortality rates in calves during their milk feeding period represent an enormous waste of genetic potential in the dairy herd as well as cash outlay (Moran, 2002).
There are easily quantifiable benefits in having more newly calved heifers available to replace older unprofitable cows, as heifer and reproductive management improve (Morton et al., 2003).These benefits are: • 1 to 2% more first calf heifers for every month reduction in age at first calving • 3 to 5% more first calf heifers for every 10% reduction in calf mortality • 2 to 3% more first calf heifers for every month reduction in inter-calving interval Replacement heifers are bred to allow for the culling of cows no longer suitable for the milking herd.Good heifer management is then essential to provide sufficient animals for this to occur on a regular basis.The proportion of heifer calves that survive and grow well enough to become replacements depends on the many factors described above.These can be quantified as the proportion of • Milking cows that actually conceive (the conception rate) • Those that produce a live calf (namely, do not abort during pregnancy or suffer neonatal death) • Those that are heifers (usually 50% of the viable calf drop, except when using sexed semen) • Those that survive until calving (namely, do not die during milk rearing and post weaning) • Those that conceive as maiden heifers • Those that are suitable as milking cows in the herd (for example are not culled because of poor temperament or because of lengthy illness) STOAS (1999) compared reproduction and calf survival in two rearing systems to calculate their relative replacement rates for a dairy herd with stable stock numbers (in Table 5).System A measures could be considered as a set of Key Performance Indicators.
Assuming cows remain in the milking herd for four to five lactations, 20 to 25% should be replaced each year.From Table 5, the supply of 36% heifers from System A allows for the sale of young breeding stock or a higher culling rate to better address genetic improvements in the herd.Only one in every six or seven cows could be replaced annually in System B, which would hardly be enough to maintain herd numbers, let alone allow for much genetic selection.
With high ages at first calving (>30 months) and long inter-calving intervals (>15 months), it is very difficult to increase herd size through natural increases.That is why it is so important to seek the underlying causes of herds with high percentages of dry cows or a high proportion of heifers to cows.The most likely cause is poor feeding management but there could be others, such as disease, heat stress or simply poor reproductive practices.
Farmers should aim to rear 20 to 25% of their milking herd each year as replacements, to calve down for the first time by about two years of age and produce at least five calves during their productive life.Realistic target for tropical dairy systems (Moran and Tranter 2004) are: • Calf mortality to weaning, 4 to 6% • Heifer wastage rate from birth to second calving, 20 to 25% • Live weight at mating, 250 to 300 kg • Live weight at first calving, 400 to 500 kg (depending on breed type) • Age at first calving, 28 to 30 months.

STRATEGIES FOR IMPROVING YOUNG STOCK MANAGEMENT
High calf mortalities on tropical dairy farms should not be accepted as "the norm" and need to be addressed by extension programs targeting the following aspects of management: • Ensuring effective transfer of passive immunity to all new born calves through good colostrum feeding management • Housing to minimise transfer of disease between calves, such as using individual pens or calf crates • Good hygiene of calf pens and feeding equipment • Providing clean drinking water within their first week of life • Milk feeding regimes to encourage early rumen development through restricting access to limited quantities of raw milk or quality calf milk replacers so calves will seek out solid feeds • Appropriate feeding management based on formulating palatable calf concentrates to supply adequate energy, protein and fibre • Close attention to animal health to minimise the incidence of calf scours, pneumonia and other diseases prevalent in many traditional calf rearing systems • Adequate climate control to reduce the adverse effects of heat stress on calf performance • Good record keeping so farmers can more easily identify susceptible calves and quickly treat potential problems • Ensure easy access to veterinarians and other animal health specialists to develop and implement effective animal health programs

Good colostrum feeding management
For each hour delay in colostrum feeding in the first 12 h of life, the chance of a calf becoming ill increases by 10%.
The key to ensuring new born calves have adequate transfer of passive immunity, through absorbing sufficient antibodies into their blood stream, during their first day of life, is to follow the principles of the 3 Q's, namely: i) Quality; providing good quality colostrum ii) Quantity; ensuring calves ingest sufficient antibodies iii) Quickly; timing the first feed to ensure efficient absorption from the gut to the blood.These principles can be quantified as follows: • Use colostrum from mature cows that produce less than 8 L during their first milking • Use only first milking colostrum • Feed 4 L to large calves and 3 L to smaller calves at first feeding • Feed colostrum as soon as possible, at least within the first 3 h after birth • Do not let calves suckle their dams

CONCLUSION
Poor heifer management is a major problem in many (if not most) small holder dairy farms throughout Asia and other tropical countries.Young stock receive insufficient attention because they do not generate income for many months.In addition, their first three months are the most expensive period in the life of any dairy cow and many farmers are just not prepared to invest in the calves' future.A low calf mortality rate indicates that early milk rearing practices are adequate and it provides greater opportunity for economic and genetic improvement in the herd.When a calf or heifer dies, there are fewer opportunities for culling unprofitable cows.

Table 1 .
Details of studies documenting calf mortalities in tropical dairy farming areas, listed in decreasing level of pre weaning calf mortality rates

Table 1 .
Details of studies documenting calf mortalities in tropical dairy farming areas, listed in decreasing level of pre weaning calf mortality rates(Continued) CCI = Calving to conception interval; AFC = Age at first calving; CI = Calving interval.

Table 2 .
Bede, 2008)of grazing system on heifer rearing and herd dynamics on 987 farms in the Kenyan highlands (study reported byBede, 2008)

Table 3 .
Nettisinghe et al., 2004) to an AI heifer rearing scheme in the Western Province of Sri Lanka (study reported byNettisinghe et al., 2004) Rs = Sri Lankan rupee.

Table 4 .
Payne, 1967)of Friesian cross calves from 3 to 12 months of age under various rearing systems (study reported byPayne, 1967)

Table 5 .
Measures of reproduction and calf rearing to produce replacements for a stable dairy herd on 2 rearing systems (A and B) in the tropics (data reported bySTOAS, 1999)