Applicability of National Research Council calf feeding standards to a dual purpose system in Venezuela

Restricted suckling of cows by their dams is a characteristic of dual purpose systems extensively used in Tropical America. Suckled milk constitutes the main feed during the first weeks of life and is slowly replaced by solid feed until weaning. The applicability of the National Research Council (NRC) feeding standards in these conditions was evaluated using the information obtained in five experiments where feed intake and animal response were known.

The results showed that recommended energy and protein requirements do not match with estimated intakes. The use of "NRC feeding standards" is further complicated by variable milk intake and quality of solid feeds supplied in dual-purpose herds.

The data from the five experiments were organized to relate feed consumption to expected live weight gains, as the basis of a practical guide to help decision making by farmers.

Keywords: Calves, dual purpose, feed intake, feeding standards

Who needs feeding standards? It was the title of an impacting paper by Jackson (1981) to demonstrate the inadequacy of feeding standards, derived in industrialized countries, when applied to the conditions in most developing countries with insufficient amounts of feeds of the quality needed to achieve maximum performance. The relevance of feeding standards for dual purpose cattle production systems used in lowland Tropical America is also questionable, since the intake and quality of grass ingested, the main component of the diet, is variable and unknown (Combellas 1998). Feeding systems were defined by Blaxter (1980) as a series of methods whereby diets and rations could be formulated for livestock to sustain a particular level of productivity from a wide variety of feeds. This approach assumed that substitution values of feeds could be computed from data on intake and feed analysis. However, these conditions are rarely satisfied in traditional cattle production systems in the lowland tropics. Awareness of these difficulties was what motivated Preston and Leng (1987) to develop a more appropriate approach based on ruminant digestive physiology, which stressed the need first to satisfy the nutritional needs of rumen microbes and then to provide supplementary "bypass" protein, the amount of which would be determined by establishing response curves to the available source of bypass protein and selecting the point on the curve according to the cost of the protein resource and the value of the live stock product.

The present study documents an attempt to validate the nutrient allowances recommended in "Nutrient Requirements of Dairy Cattle" (NRC 2001), when applied to the typical calf rearing system employed in "dual-purpose" cattle production systems in Tropical Latin-America (Alvarez et al 1980). The NRC (2001) standards were chosen because of their wide diffusion in our continent, but the principles underlying other systems used in temperate countries are similar.

NRC (2001) standards are oriented to feeding systems based on: milk or milk replacer; milk and starter or milk replacer and starter and post weaning. By contrast, calf feeding in a dual purpose system is based on restricted suckling of a variable amount of milk with a high fat content, concentrates based on by-products of agriculture or the feed industry and forages of variable quality. In this system, feed intake during the initial weeks of the life of the calf life changes from suckled milk alone to a combination of variable proportions of suckled milk and solid feeds.

The information accumulated in a series of experiments with restricted suckling was tabulated so as to relate feed consumption (in terms of metabolizable energy [ME] and crude protein [CP] to live weight gain. These data were then used; (i) to relate recorded intakes with those predicted from NRC (2001); and (ii) as a guide to feeding and management of calves in dual-purpose systems.

Intakes of milk, forage and concentrates and rates of gain in live weight were compiled from records of 87 calves used in a series of 5 experiments designed to evaluate different regimes to stimulate milk ejection in a restricted suckling system. The experiments were carried out in the Faculty of Agronomy, Maracay, Venezuela, from 1998 to 2002 using calves born in a herd of ? to ¾ Holstein x Brahman cows. The calves were housed in individual pens, except during the first three days after calving when they were kept with their dams. The calves were suckled by their dams in the pens for about 30 minutes after the morning milking and most of them were taken to the milking parlour to stimulate milk ejection. Milk consumed by calves was estimated weekly from the second week, by weighing them before and after suckling. A milk sample was taken just after suckling started and its fat content was analyzed. A concentrate mixture and forage were offered in the morning from the second week onwards and intakes determined weekly by measuring the difference between amounts offered and refused. The forage was offered ad libitum but the concentrate was limited to 1 kg/day, a value reached usually at between 90 and 119 days of age of the calves. The concentrate DM had 18.5 % and an estimated ME content of 12.6 MJ/kg, and was elaborated mainly with wheat middling and extracted maize germ meal. The forage consisted of chopped Cynodonnlemfuensis, Brachiaria mutica and Digitariaswazilandensis, and was offered in amounts that would lead to a refusal of at least 10 %. On average the CP was 7.5 % while the estimated ME content was 8.4 MJ/kg (both on DM basis).

Estimation of ME and CP requirements and intakes

ME and CP requirements were calculated weekly for average values of all the 87 calves using the formulas described by NRC (2001) to compute tables of requirements for animals fed milk or milk substitute and starter and details are given below.

Daily ME requirement (Mcal) is the sum of ME for maintenance and ME for live weight gain, derived from NE requirements for maintenance and gain corrected by the efficiency of use of ME for maintenance and gain, according to the type of feed:

ME = [(0.086 LW^0.75) / 0.825] + {[(0.84 LW^0.355 x LWG^1.2) x0.69] / 0.625}

Where LW is live weight in kg and LWG is in kg/day.

Daily crude protein requirements (g) are derived from the apparent digestible protein (ADP) of each feed:

CP = ADP/ 0.8645

ADP requirement is partitioned into components of maintenance and gain: (E) endogenous urinary N, (G) amount of N in LW gain and (M) metabolic fecal N per kg of dry matter consumed times daily DM consumption (D), corrected by the biological value (BV) of each feed:

ADP = (6.24) [1/BV (E + G + M x D) - M x D]

Values for M, S and W are estimated to be:

VB: 0.764;

E = 0.2 * LW^0.75

G = 30 g/kg LWG

M = 1.9 g/kg Dry Matter Intake (DMI)


Requirements were computed for calves of 35, 63, 91 and 119 days of age, using the average DMI and LW at these ages and LWG calculated as the slope of the regression of LW against time (days) during the previous 4 weeks. Fixed ages instead of fixed weights were used instead of figures derived from interpolation. Milk DMI was estimated by adding fat content (analyzed) to solids non fat (assumed to be 9 % from measurements in one of the trials). Total DMI was obtained by summating milk, concentrate and forage DMI.

ME milk intakes were calculated using the equation given by NRC (2001) to estimate the energy of milk from its fat content assuming a conversion to ME (q) of 0.93.

A CP content of 3.5 %, obtained in one of the trials, was used to estimate CP intake from milk.

A table was elaborated to present the information relating feed intake and selected ranges for rates of LWG. Average values for intake of milk, concentrate and forage corresponding to the five selected ranges of LWG were calculated at four week intervals. LWG ranges from birth to weaning were set at 0.10 kg/day intervals from 0.26-0.35 to 0.66-0.75 kg/day, based on the data of the 87 calves used in the validation process. LWG means within a range were close to 0.3, 0.4, 0.5, 0.6 and 0.7 kg/day. Weekly intakes at 4, 6, 12 and 16 weeks were calculated as the means of those weeks and the previous and following week to obtain a more accurate estimate, for example, week 4 milk intake was the mean of weeks 3, 4 and 5 milk intakes.

The relationship between ME and CP intakes and NRC requirements are presented in Figures 1 and 2 and data to support them is given in Table 1. The requirements for milk and starter are given in tables up to 60 kg LW calves and were calculated here up to 86.7 kg LW. Values in Figure 1 are given in MJ in accordance with standard practice.







Milk ME requirements of a calf consuming milk by sucking from its mother are lower than for calves consuming solid food for two reasons: because suckled milk passes directly to the abomasum through the oesophageal groove, and there are no losses of methane or fermentation heat and the constituents can be used directly (Ørskov and Ryle 1990); and because the efficiency of use of ME to NE is elevated in high energy feeds such as milk. In Figure 1, observed ME intakes for calves reared by restricted suckling, are higher than calculated (NRC) requirements and the application of the NRC (2001) standards would result in lower rates of LWG than expected from the standards.

Several factors can explain these differences. Young calves in dual purpose systems ingest the high fat milk remaining after milking and it was shown by Thivend et al (1980) that feeding of high fat milk replacers reduces protein retention, suggesting that dietary lipids are catabolized less easily in animal tissues than glucose, which would result in an over-estimate of the ME in tissue growth for calves in a restricted suckling system (as in Figure 1). In older calves consuming a high proportion of solid feeds, bias could arise from over-estimate of ME in tissue growth and the use of ME conversion factors derived from adult animals. The efficiency of utilization of medium quality forages and "by-product" concentrates by calves may also be over-estimated. Use of feeds of lower feed energy "quality" also increases energy requirements, because the efficiency of use of ME for NE is directly related to feed quality.

The observed CP intakes for calves on the restricted suckling system coincide with requirements established for milk or milk replacer in young calves at 35 days consuming a high proportion of milk (Figure 2), but afterwards the observed intakes are higher and the difference increases as milk consumption decreases. As with energy, CP requirement of a calf receiving milk by suckling is lower than when animals are fed milk or milk replacer from a bucket, because of the efficient "bypass" mechanism of the oesophageal groove in the suckling system. By contrast when milk enters the rumen as frequently happens in artificial rearing systems, part of the protein is fermented with resultant losses of ammonia through the rumen wall.

The restricted suckling process in dual-purpose systems maintains the efficient bypass of the milk to the abomasums, while solid food consumption increases slowly with age and is digested in the rumen. This system is more efficient in the utilization of milk but less efficient in use of solid foods, compared with the NRC (2001) system where the concentrate "starter" is recommended to be of "high" quality (eg: maize and soybean meal). By contrast, in the dual-purpose system the concentrates are usually derived from crop by-products which have lower "net" energy and ADP values.

Applicability of standards

Published energy and protein standards such as those in NRC (2001) are not applicable to our conditions as was shown in the previous section. The question to be answered concerns the possibility to modify and adapt them by the use of more precise values to describe the nutritional merit of tropical forages and agro-industrial by-products. In theory it is possible to estimate more accurately the nutritional quality of feeds, and the true energy deposition when high-fat milk is consumed by restricted suckling. In practice, two problems arise. The first is the variation of the energy and protein value of forages which are usually fed fresh and at different stages of growth, and their influence on ME requirements as the calf gets older and is more dependent on them. The second is the ignorance of the amount of milk consumed with restricted suckling, which is basic information required to estimate energy and protein intakes from milk. Consequently adequate, and at the same time, practical feeding standards for calves in dual-purpose system is not likely to be available in the near future.

Feed intakes of calves in these trials

It is apparent that the NRC (2001) standards do not fulfil the objectives of a feeding system such as is described in the introduction. For these systems, the data in Table 2 which are derived from observed feed intakes, can be used as a guide to calf feeding management in dual-purpose herds in the tropics. Factors to be taken into account in applying these suggested allowances are the variation in the available feeds and the fact that milk consumption by suckling can vary widely. It decreased with age and according to the management of the suckling process, such as suckling frequency; method of cow stimulation at milking and number of teats milked.

Table 2.   Live weight (LW, kg), milk intake (kg/day) and concentrate and forage DM intake (kg/day) observed over five ranges of live weight gain (LWG)



In practice, as indicated in Table 2, LWG increases slightly with age while milk consumption decreases and then tends to stabilize. Intakes of forage and concentrate are almost negligible during the first month and increase exponentially towards weaning. The data show clearly that milk intake is the major determinant of LWG, and that other components of the diet vary to a much smaller extent.

Conclusions

• Feeding standards for energy and protein requirements of calves, as recommended by NRC (2001), are not appropriate for dual-purpose systems in Tropical America where restricted suckling is standard practice. Their adaptation by the use of more precise estimates of the "quality" of the energy and protein components of tropical feeds is of no practical application, because of the wide variations in composition of forages and concentrate ingredients that are locally available; and the high variations in the amount of milk consumed.
• A more appropriate guide to calf feeding management in dual-purpose herds is to use data for feed consumption and expected live weight, derived from practical observations.

The authors express their gratitude to FONACIT for the financial support (Project No. S1-2002000409)

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