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Botanical sampling as a meansof identifying the componentsof sheep's diet in tussockgrasslandJ. M. Hercus aa Farm Advisory Division, Department ofAgriculture , Christchurch , New ZealandPublished online: 05 Jan 2012.

To cite this article: J. M. Hercus (1963) Botanical sampling as a means of identifyingthe components of sheep's diet in tussock grassland, New Zealand Journal ofAgricultural Research, 6:1-2, 83-89, DOI: 10.1080/00288233.1963.10419323

To link to this article: http://dx.doi.org/10.1080/00288233.1963.10419323

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BOTANICAL SAMPLING AS A MEANS OF IDENTIFYING THE COMPONENTS

OF SHEEP'S DIET IN TUSSOCK GRASSLAND

By J. M. HERCUS

Farm Advisory Division, Department of Agriculture,

Christchurch

(Received 29 March 1961)

ABSTRACT

Several features of the environment and management of tussock grassland add to the difficulties of using simple observation to determine the diet of sheep. The flora is a large one, and in response to variations in soil, aspect, altitude and management the vegetation on any grazing block consists of a mosaic of plant communities. Grazing intensities are low and grazing periods long, and the sheep are nomadic and easily disturbed. A century of occupation has led to some conclusions as to the relative palatabilities of a few of the more abundant species but the picture is far from clear and there are several apparent contradictions. This article describes a method of botanical sampling to identify what sheep eat in tussock grassland. The technique involves sampling the area concerned with a 1 ft diameter ring, all species inside the ring being recorded and each rated according to its degree of defoliation.

INTRODUCTION

83

The present area of tussock grassland has been used for extensive sheep grazing for almost a century. Following occupation, there came a period of trial and error to establish some measure of equilibrium between animal numbers, rabbits as well as sheep in many cases, and the vegetation and climate of the various areas. Within this period, a broad pattern of management was evolved. The main grazing restriction was an altitudinal one, the sheep being held above the normal winter snow-line for the midsummer to autumn months and below this level for the rest of the year. In many cases, there has been further grazing control by the provision of separate winter blocks for hoggets, ewes, and dry sheep. However, under all intensities of subdivision of hill country, the animals can graze a variety of slopes, aspects, and altitudes on anyone block.

Some 210 native and 50 introduced species have been recorded in the tussock grassland, and it is not uncommon to find 50 of these on a single hillside or valley floor. The actual plant association varies with such factors as slope, aspect, altitude, soil, and previous grazing and burning history. The result is that on any block of tussock country the sheep have access to a range of plant species and communities.

N.Z. J. agric. Res. 6: 33-9

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84 I dentification of sheep's diet on tussock grassland

This range of species is paralleled by the range of their palatability: a few appear to be consistently grazed; some only at certain times, stages of growth, or on particular aspects; and many seem to be rarely if ever eaten. General observation has indicated the part played by several of the more abundant species, but there is a large number of plants whose palatability is either not known, or about which the evidence is contra-dictory.

OBJECTIVE

The problem was to find a method of identifying what sheep eat on tussock grassland. It was primarily a qualitative study, and although plants were rated according to their degree of defoliation, no attempt was made to measure the contribution of individual species.

It was felt that an important way to improve both plant cover and yield would be to identify the most abundant palatable species. By studying their requirements for optimum growth and spread, a system of management could be devised to favour these valuable plants. The aim of this approach is to adjust the requirements of the stock to encourage desirable species, in contrast with the present system which requires the vegetation to fit into a rigid pattern of grazing management.

METHOD

A 1 ft diam. wire ring was dropped at pre-determined intervals along a series of traverses covering the area being sampled. For sub-sequent recordings more or less similar traverses were followed. Plant communities that were obviously different were sampled separately, as also were different aspects of the same association. This separation applied only to appreciable variations in cover and aspect, wherever it was thought there might be changes in the intensity or pattern of grazing which should be recorded.

Reasonably homogenous aspects or areas of more than 10 acres were sampled at an intensity of about 4 recordings per acre, but smaller or more variable areas were recorded with 7-8 samples per acre. In the experiment to be described, 90-100 samples were taken on face (block) A of about 20 acres, 55-65 on the 12 acres of face B, and another 55-65 on face C, of roughly 8 acres. These intensities were lower than had been planned but were adopted for the reason that they allowed complete sampling of the trial area by 2 observers in the one day.

All species within the ring were recorded but no note was taken of the frequency with which anyone occurred. Each species was closely examined and rated on a 0-4 scale according to the severity of grazing.

Two conditions have been found where this method of sampling has distinct limitations. The first is on steep depleted hillsides, and the second is in long dense vegetation. In the former case the ring tends ro slide until it meets an obstruction or reaches its angle of repose. In the trials so far, the areas the ring would not sample were almost invariably bare.

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J. M. HERcus 85

Where there is rank growth, the ring may not fall fiat, or where it does it may rest too far above ground level to permit easy observation. This condition was met only rarely on the areas examined.

THE EXPERIMENT

The main trial was done on an 80 acre experimental block on Mr D. Kane's property near Hawea Flat, in the Upper Clutha basin of Central Otago. The area ranges in altitude from about 1,300-2,000 ft, and roughly two-thirds of it faces north to north-west. It is typical of much depleted winter country in this district.

In 1949 the Department of Agriculture put down a series of oversowing trials on part of this block. The area is divided by a small central gully into 2 more-or-Iess comparable halves, each with a dominant sunny face and a smaller area of semi-shady and shady aspect. The southern half was left as a control, and the remainder subdivided into 3 on the basis of aspect: face A was the depleted north face, containing the greatest area and the most severe depletion; face B had a westerly aspect, and was moderately depleted; and face C was the small area of dark face. Using r~sults from earlier plot trials in the district, different seed mixtures-mostly of introduced species-were broadcast over these 3 aspects in March 1949.

By 1955 the plant cover was made up very largely of rejuvenated resident species on the lower sunny slopes, but there was still more than 50 per cent bare ground. The most seriously depleted faces showed little improvement. Wherever there was some soil and protection, oversown species were giving fair to good ground cover.

Initially, the only grazing was by rabbits, but, with their virtual elimination in 1951, sheep were carried from MayIJune until September each year from 1953.

In the second week of June 1955, 54 fine-wool ewes were put on the block. Lambing commenced in mid-July, and, because of the weather, the type of feed, and the lack of permanent water it was decided to take them off at the end of July. Ten days later, 120 hoggets were put out, and these remained there until the end of September. The block was then spelled until April 1956, when 80 hoggets were put on until late September. The sheep came from Mr Kane's farm.

In 1955, approximately fortnightly sampling was carried out between June and September. The following winter the area was sampled only twice, in July and October. On most occasions there were 2 observers, and the same aspects were not always sampled by the same person. The oversown portions and the untreated control area were both examined, but results are given for only A, B, and C faces, where the wider range of species was present and where the sheep could choose from among native and introduced perennial plants.

A similarly depleted hill block in a different locality was sampled in July 1955 and in the same month in 1956. Two other winter-grazed

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86 I dentification of sheep's diet on tussock grassland

areas were also examined in 1956. The results from these 3 sites are not given here, but for the species, associations, and aspects common to all areas they confirmed the pattern of selection shown on Kane's block.

RESULTS

Sixty-six species of grasses and herbs were recorded on Kane's block, 55 of them having been present prior to March 1949, and 11 oversown. Certain species were well distributed over particular aspects, e.g., Agropyron scabrum (R.Br.) Beauv. on all but the steepest slopes of A face, and Arrhenatherum elatius (L.) Beauv. ex J. et C. PresIon C face, while others tended to be concentrated into small patches in response to differences in soil or exposure. As well as this variation in type of distribution, there was a considerable range in the frequency of occurrence among species.

The sampling intensity used was inadequate to allow evaluation of the role of species which occurred infrequently or which were of restricted distribution, and results are given for only those plants present in 10 per cent or more of the total recordings per aspect. There are palatable productive species in the under 10 per cent group, but at the present stage it is felt that their apparent limitations of habitat, competitive ability, or poor seeding make them of lesser importance. Palatable plants occurring in 10 per cent or more of samples are likely to constitute the greater part of sheep's diet, and it is within this group that manipulations of management could be expected to have rapid worthwhile results.

Species eaten

The objective was to find out which species were grazed and which were not, and the results can be summarised as follows:-

Face A. A total of 47 over IV per cent, and. of these

Face B. per cent or more of :S"'llll-',n.

with face A.

was recorded, 12 in frequencies of 8 were consistently grazed.

were present, again with 12 in 10 of these were grazed, 6 in common

Face C. Forty-two species were recorded, 11 of them in 10 per cent or more of the samples. Four of these latter were regularly eaten, 2 of them in common with A and B faces.

Taking these 3 blocks together, there were 21 species present in at least 10 per cent of the samples in at least one block, and 12 of these were found to be regularly grazed.

For most of these 12 preferred species, it was possible to fit to each a linear trend in the percentage eaten against time through the grazing season. The occurrence of this linear trend means that the species were subject to regular grazing pressure. For example, the results for Danthonia buchanani Hook. on A face in 1955 were:

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J. M. HERCUS 87

Date of sampling (1955) 29/6114/714/8 15/811/9 17/10

Percentage of samp~e:~:-~hich plants -- -1'---1 . 1

1

----1 -_ ... showed signs of being grazed 21 : 42 58 77 67 81

(The trend is positive and significant at the 1 per cent level.)

In the second winter, only 2 recordings were taken, but these suggested the same sort of trend, e.g., D. b-uchanani on A face, in 1956.

Date of sampling (1956)

Percentage of samples in which plants had been grazed

11/7

88

(This difference is not significant by X2 test.)

10/10

96

The fact that for several species the differences between the 2 1956 readings failed' to reach significance-as is shown in the table given above for D. buchanani-may be because:-

(a) There is a much higher percentage grazed in the first 1956 recording compared with those obtained at the same period in 1955. There are 2 factors involved here, the first being the different number of sheep being grazed, and the second the fact that the sheep were put on the area in April in 1956, 2 months earlier than in the previous year.

(b) The 2 readings in 1956 give a less sensitive measure of trend than do the 6 taken in 1955.

Observation suggested that because grazing commenced much earlier in 1956, and due also to the milder, more prolonged autumn that year, the sheep spent much more time on Band C faces, at least up until July, than they had done in the previous year. This was confirmed by the botanical sampling, as is shown by the figures for percentage of samples grazed for the following species:

Dates of sampling 14/7/55 11/7/56

Arrhenatherum elatius, Face C o 69

Anthoxanthum odoratum, L Face C 20 40

Danthonia pilosa, R.Br.Prodr. Face B 14 67

The figures obtained appear to give an estimate of which of the reasonably abundant species have been grazed, but they do not show how much each species has contri'buted. At each sampling, species

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88 Identification of sheep's diet on tus.IOck grassland

present inside the ring were rated according to the severity of grazing. This index was descriptive of the individual species and bore no relation to their relative production or contribution to the diet. It was used to try to distinguish consistently grazed plants from those eaten only accidently or occasionally. However, with some knowledge of the growth habit and relative yield of severa! of the species, a very general sort of quantitative relation could be postulated.

These results confirmed those obtained by general observation, but served to allow a more precise definition of the role of many of the reasonably abundant plants.

Occurrence of species

The percentage occurrence figures showed that most of the perennial species remained at much the same levels during and between each sampling period.

Dates of sampling

Danthonia Buchanani, Face A.

Arrhenatherum elatius, Face B.

1955 1956

, 29/61 ~4/714/8115/81l/917/10 1117110/10

I 32 33 33 I 31 I 33 I 31 II 27 I 23

42 I 42 43 I ~~--I 44 II 42 I 48

(The figures refer to the percentage occurrence of the species in the to-tal samples per face.)

The lack of haphazard variation in these results suggests that the sampling was adequate for the reasonably abundant species.

However, 2 species showed a regular increase in percentage occur-rence, due to winter germination, and in these cases figures for percent-age grazed must be related to this increase.

Dates of sampling

Crepis capillaris (L.) Wallr. Face A

Vulpia dertonensis (All.) Volk. Face A

1955 i

129/6 14/714/8 \ 15/8 \ 1/9

I I 33 37 39

I

46 45

0 0 I

21 44

1956

\ 7/10 11/7110/10

52 18 36

51 70 85

(Figures refer to percentage occnrrcnce of these spe::ies.)

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J. M. HERCUS 89

In the case of Vulpia dertonensis, the large difference between the 2 July samplings may be some measure of cover improvement, but an important factor was the prolonged mild autumn of 1956 which allowed rapid and early germination.

CONCLUSIONS

This method of botanical sampling can be used to give information as to what sheep eat at any time of the year. The technique was developed for use under extensive grazing conditions, where the animals have access to a range of species, aspects, and plant associations, and where the vegetation is of the tussock type, but not rank and completely multi-storied. The method and the sampling intensity appear suitable to permit an evaluation of the part played by the more abundant species. It is felt that this level of sensitivity is sufficient, at least in the early stages, as not only do members of this group make what is prob-ably the largest contribution to sheep's diet, but it is here that manipula-tions of grazing management are likely to have worthwhile results.

The technique is simple, requiring the minimum of equipment, and it can be used by a .single observer to cover large areas in a short time.

ACKNOWLEDGMENTS

The author wishes to express his indebtedness to: Mr J. C. Tothill, formerly of the Department of Agriculture, Alexandra, for much of the field work and for the summaries of results; Miss J. G. Miller and Mr N. Mountier, Biometrics Section, Department of Agriculture, for the statistical analyses and for their assistance in the interpretation and presentation of the results; and Mr D. Kane for his untiring co-operation.

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