V. Secondary production, nutritive value and use by animals

Measuring the secondary production of pasture: an applied example in the study of an extensive production system in Mali**

M.S. Dicko-Touré

Nutritionist on the ILCA/Mali Sahel programme, Niono, Mali

Introduction

1. Summary of methods for measuring secondary production

1.1 Assessing palatability

1.2 Measuring the quantity and quality of the forage ingested

1.2.1 Faecal production

2. Example of simultaneous application of different methods for measuring secondary production in the study of an extensive livestock production system

2.1 The herd and the environment

2.2 Methodology

2.3 Results

Bibliography

Introduction

Calculating secondary production, which may be defined as the profit derived by livestock from a given pasture, has been and still is throughout the world one of the main preoccupations of research in applied agronomy. It is justified to an even greater extent in intertropical Africa and Madagascar, where over 150 million cattle, 240 million small ruminants and 12 million camelides (FAO, Production Yearbook, 1978) live under extensive conditions using the forages provided by natural rangeland as their only feed resources.

A number of methods for measuring the factors affecting secondary production (primary production of pasture; palatability of plant species, quantity, digestibility and nutritive value of the forage consumed by the animal) have long since been put forward. For example, Henneberg and Stohmann (1860-1864) defined a method for assessing apparent digestibility. Edin (1918) and Wildt and Reid (1950) respectively suggested the utilization of chromic oxide, silica and chromogen as indicators in the determination of faecal production and the digestibility of the forage ingested. Despite long-standing concern with this subject, a complete understanding of the factors involved in secondary production has not yet been achieved in a satisfactory way, owing to the impossibility of carrying out any direct investigation into what the animal selects from the pasture.

The aim of this paper is not to describe the various methods of measuring secondary production. These are known and frequently reported in a number of first rate reviews (Riviere, 1975; Schneider 1975; Cordova, 1978; Menke, 1978). Rather, the intention is to summarize them briefly and then describe a new methodology that combines in a single protocol a combination of these techniques that are usually used separately.

1. Summary of methods for measuring secondary production

1.1 Assessing palatability

The factors affecting palatability vary in both time and space and are also linked as much to the animal as to the plant.

According to the species animals have feed preferences that lead them to make greater or less use of this or that stratum of vegetation. Thus, in the case of domestic animals, goats and camels, which are browsers, make greater use of the ligneous stratum than do cattle and sheep. To this observation should be added the fact that the physiological or nutritional condition of the animal makes it more or less selective.

On the plant side: "the palatability of a taxon varies according to its stage of development, its relative abundance on the rangeland, its chemical composition and its organoleptic qualities. Some plants are consumed when young and ignored on reaching older stages. Others are more palatable when scorched and withered when green. It is also known that the species composition influences palatability: the more a taxon is abundant, the less it is sought after by animals, with the exception of a few plants which are particularly appreciated under any circumstances. Essential oils in some plants are a factor restricting the utilization. The cellulose and mineral contents also play important roles in plant selection". (Le Houérou, 1980).

Methods for assessing palatability do, however, exist:

a)collecting information from herdsmen;

b)collecting information via behaviour studies or observations on animals;

c)collecting information by examining and analysing the forage selected by animals equipped with a rumenal or oesophageal fistula;

d)using cellulose and mineral contents as indicators.

All these data, linked with the opinions of herdsmen, the accuracy of observations, individual variations between animals and plants, make the task of assessing palatability a delicate and very often a subjective operation, always risky to generalize.

1.2 Measuring the quantity and quality of the forage ingested

Assessing the quantity and quality of forage consumed is also an arduous task. It is nevertheless important, since the main factor determining animal performance is the digestible proportion of the feed consumed.

Certain factors linked with the animal itself, such as species, age, level of production, physiological condition and others linked with the feed, such as membranous glucide or nitrogenous contents, and yet others linked with the environment, such as forage and water availability, and the air temperature, also influence ingestion and the digestibility of feeds.

Consumption from pasture is determined indirectly by using the relationship between ingestion, digestibility and faecal production:

Weight of forage consumed = weight of faeces excreted × 100
                                                     100-digestibility

1.2.1 Faecal production

Faecal production can be measured either by collecting all the faeces of animals equipped with bags, or else by ingestion of indigestible substances called indicators and determination of the share of the latter in faeces. The weight of faeces produced is, in the second case, given by the formula:

Production of faeces (g of DM)= weight of indicator ingested
                                                  weight of indicator per gram of faecal DM

Generally speaking, on pasture the indirect method (using indicators) is the one most used, total collection of faeces being regarded as laborious, too time-consuming, expensive and impractical in extensive production systems (Brisson, 1960; Corbett, 1960; Schneider, 1975).

In contrast, we were able to establish that total collection of faeces from extensively raised male cattle is quite possible and is even easy. We have designed a special bag and developed a way to attach it to the animal. These bags are used in the study of traditional systems in Mali. We recorded:

a) no loss of time: one man-hour for the collection of faeces from each individual animal, which is very far from the 70 man-hours estimated by Kartchner (1975);⁽¹⁾

b) no high cost, since using indicators would have meant sending the faeces to another country to determine the chromic oxide content (the cost would have been 10 times higher);

c) no excessive labour, discounting the two or three first days (the time required for the animal to adapt to carrying the bag). The lavour aspect of the operation is greatly diminished, probably owing to the fact that under traditional systems the animals live at very close quarters to the herdsman and the other members of his family.l2) Usually they use the immediate surroundings of the herdsmans's but as a resting area and are thus neither afraid nor disturbed by the frequent comings and goings of men around them. They can thus be easily handled.

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⁽¹⁾Our measurements of time deal with the time needed to accustom the animal, attach the bag before departure on the rangeland and the reverse operation when the animal comes back to the camp, Kartchner's estimation is purely subjective, however, as in our case it does not include preliminary preparation, nor sampling of faeces for analysis.

⁽²⁾ These are Fulani stockraisers and their cattle.

1:2.2 Digestibility

1.2.2.1 Methods using indicators

Indicators can be normal components in the ration (natural indicators) or else can be added to it (artificial indicators). Digestibility is calculated according to the following formula:

                           Cont. of ind. Cont. of ind.
Digestibility of in faeces

forage grazed = in feed × 100
                          Cont. of ind. in faeces

The best known natural indicators are lignin, silica and chromogen, the later being a group of pigments contained in herbages. Artificial indicators include chromic oxide, iron oxide, barium sulphate, titanium oxide and radioactive traces. Natural indicators are more commonly used for estimating digestibility, while artificial indicators, of which chromic oxide has been the most studied, are used for determining faecal production.

1.2.2.2 Methods using the chemical composition of faeces

These methods are based on the relationship between the content of faeces in certain ingredients, known in these cases as the faecal index, and the digestibility of dry matter and/or organic matter:
Digestibility of forage grazed = concentration faecal index. 
The faecal index consists of nitrogen, lignocellulose and chromogen.

This method is relevant for the study of pastures since it enables the digestibility of a ration to be evaluated without requiring the chemical determination of the latter. However, its relevance is limited to certain conditions (Cordova, 1978), since it appears that the relationships between digestibility and the faecal index are not causal links but mere concomitance (Scaut, 1975). Faecal index techniques are only applicable a priori within a given plant population, and their parameters depend to a certain extent on the animal used and the plane of nutrition (Scaut, 1955).

1.2.2.3 Methods using in vitro analysis of digestibility

Samples used for this technique are gathered by means of animals equipped with oesophageal or ruminal fistulae. Others originate from forage which the animal has been observed to graze or browse. The analysis method most commonly used is the two-time method of Tilley and Terry (1963).

Of all the estimates of ingestion from pasture, it appears that the best data have been obtained when faecal production is determined using the total collection method and when digestibility is measured by in vitro analysis of samples of the forage grazed (Cordova, 1978).

2. Example of simultaneous application of different methods for measuring secondary production in the study of an extensive livestock production system

The aim of this methodology, which was applied in nutrition studies carried out by the Sahel programme of ILCA at Niono in Mali, is to carry out simultaneous evaluation of a number of parameters of the three important factors environment, management and feed, which together influence the plane of nutrition and consequently animal performance.

2.1 The herd and the environment

The herd consists of 90 head. The management method, resembling that used for most cattle in the Office du Niger zone ⁽³⁾ is semi-sedentary with a range of influence extending in a circle with a radius of about 30 km round the irrigated perimeter. From the natural grazing land where the animals remain from July to October, they arrive on the millet fallows and fields in November and December, then progress to the rice plots in January to April, before returning to the rice and millet fallows and fields which at this period constitute a holding area, until the cycle is begun over again in July with the onset of the rainy season.

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(3) The Office du Niger is a development organization for irrigated cropping in the dead delta of the Niger in Mali.

All the measurements described below were carried out over a period of five consecutive days at the end of each month on ten animals. The study phase involved all twelve months of the year, so that an evaluation of the different types of pastures could be obtained, as well as a better knowledge of the effects of bioclimatic factors.

2.2 Methodology

2.2.1 Behaviour study

The behaviour study consisted of monitoring 4 animals with an average liveweight of 250 kg. The same animals were always used and the activity of each at the time of observation (grazing, resting, ruminating, walking and watering) were recorded every 15 minutes for five consecutive days (i.e. 120 hours and 1920 observations). Details as to the nature of the forage and the position of the animal when resting or ruminating were also noted.

2.2.2 Study of movement speeds and grazing intensity

An animal selected on account of its docility was followed during the same period. While grazing, the distance moved during one minute and the number of mouthfuls taken during the same minute were measured every 45 minutes.

A sample of the forage was then collected from the rangeland used during the 1 minute period, in proportion to the observed number of mouthfuls (1 hand grab for 5 mouthfuls). This sample is the one to be used for laboratory analysis.

2.2.3 Study of consumption

The quantity ingested will be calculated according to the following formula:

DM forage ingestion = DM faeces × 100
                                   100–digestibility

2.2.3.1 Faecal production

Four animals different to those used for the behaviour study, with an average liveweight of 250 kg, are used for the total collection of faeces, over five days. The faeces are collected and weighed twice daily. After each product has been homogenized a sample is taken to determine dry matter, organic matter and total nitrogen.

Table 2
Ingestion

2.2.3.2 Digestibility

a) Determination from rumen contents

An animal weighing 370 kg is fitted with a fistula in the rumen and introduced to the herd. Over the five-day period two samples are taken, on the second and fourth day. The rumen is emptied in the morning before the animal is sent out to graze and the sample is collected after two or three hours of grazing. This method was developed by Blancou Calvet et al (1977). The material gathered by this method will be used for the in vitro determination of digestibility and for classical feed value analysis.

b) Determination from hand samples

Samples gathered by hand (cf. 2.2.2.) are dried and are processed in the same way as above.

c) Determination using nitrogen faeces as a faecal index

Digestibility will be calculated using the Lambourne and Reardon 1963 equation:

Y.MO=2.04–0.24xn +0.186xn²

Where xn = percentage of nitrogen contained in the organic faecal matter, and Y.MO = nitrogen faecal index.

Digestibility MO = Y.MO - 1    × 100
                             Y.MO

These 3 methods should enable a correct assessment of the digestibility of the forage grazed to be made.

2.3 Results

The graph and tables which follow reproduce some of the data, recorded during this study. Except for graph 1, where the results refer to a whole year, the remaining data relate to the months of January, February and March, spent on the rice plots. For example:

Characteristics of pasture during the year:

( ) = Situation in relation to irrigated perimeter

July                                      Onset of rainy season

Aug      Natural pasture        End of rainy season

            (20–30 km)

Sept

Oct

Nov        Post-harvest millet field and fallows (5-10 km)

Dec

Jan         Rice fallow

Feb         Post-harvest rice plots

Mar   Rice plots

         (0–2 km)

Apr

May

June     Irrigated rice + millet fallows and fields (0 – 5 km)

Table 1
Production of faeces

Table 3. Relationship between ingestion (I) grazing time (G) and ruminating time (R)

Month	Ingest. (DM) g/d	Duration of G (min)	Duration of R (min)	Duration of G per kg DM (min)	Duration of G per kg DM (min)
Jan	6412	476	479	74	75
Feb	6331	493	475	78	75
Mar	4622	527	383	114	83

Figure 1. Environment, management, behaviour. Seasonal variation of some parameters. (The times and distances reported are for a 24 hour period).

Figure 2. Distribution of time spent on different activities (in % of hour over a 24 hours period)..

Table 4. Relation between grazing speed and no. of mouthfuls per minute

Regression equations between mouthfuls/min and grazing speed

Table 5. Distribution of ruminating time (ru) and resting time (re). Between starting (Ru-ReST) and lying (RuLy-ReLy) positions

Month	Ru  (hours)	Ru.St.  (% Ru)	Ru.Ly.  (% Ru)	Re  (hours)	Re.St. (4 Re)	Re..Ly. (% Re)
Jan	7.76	37.4	62.6	7.15	30.8	69.2
Feb	7.92	26.0	74.0	6.46	22.5	77.5
Mar	6.38	27.8	72.2	7.07	29.2	70.8

Table 6. Distribution of grazing time (g) between different types of forage

Month	G (hours)	Browse	Millet Stubble	Rice Straw	Rice-plot regrowth
Jan	7.93	3.9	1.1	66.0	26.0
Feb	8.21	–	–	81.8	18.2
Mar	8.78	–	3.8	69.0	27.2

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