Weather and lamb mortality in a commercial lowland sheep flock

Agricultural Meteorology, 24 (1981) 237--252

237

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

WEATHER AND LAMB MORTALITY IN A COMMERCIAL LOWLAND SHEEP FLOCK J.R. S TAR R

Meteorological Office, Ministry of Agriculture, Fisheries and Food, Coley Park, Reading, Berkshire (Gt. Britain) (Received February 4, 1981;accepted March 10, 1981) ABSTRACT Starr, J.R., 1981. Weather and lamb mortality in a commercial lowland sheep flock. Agric. Meteorol., 24: 237--252. Meteorological observations were made on a lowland farm between 1976 and 1979 with a view to studying the influence of weather on lamb mortality (Purvis et al., 1979). The studies indicate that the incidence of field deaths of young lambs tends to be related to the weather "stress" experienced over the previous twenty-four hours and show some interesting features of the " e x p o s u r e " of the pastures. Trial lamb "wind-chill" forecasts are reported. INTRODUCTION

" . . . there is hardly an expense equal to the effects, both to fields and livestock, of a gnawing or destroying air" L. Bain (Q.J.Agric., 1839) Lamb mortality, during and soon after birth, is t h o u g h t to be a major source of loss to the world's sheep industry. Various studies have been made in Australia, New Zealand and Scotland (e.g., Symposium of the Scottish Agricultural Colleges, 1975). Although some figures are available from the Meat and Livestock Commission for 504 commercial lowland sheep flocks in 1970 and 1971 (suggesting a lamb birth-to-weaning mortality of 13%), and a Ministry of Agriculture, Fisheries and Food survey in the South of England for 1974 and 1975 indicated that on many farms lamb losses from birth to 6 weeks were considerable (20% or more), little detailed information has been published on the situation in England and Wales. Farm livestock can be affected by climate both indirectly, through the influence of climate upon the availability of food, and directly, as a result of exposure to the rigours of climate (e.g., Blaxter, 1962; Obst and Ellis, 1977). The rate of metabolic heat production in homeothermic animals such as lambs increases in response to an increase in heat loss to the environment and represents an a t t e m p t to stabilize body temperature. Heat loss increases as environmental temperature falls and as air movement and/or evaporation from the coat increases (Joyce et al., 1966). The environment becomes potentially lethal for the lamb when the heat loss from the lamb exceeds its " s u m m i t metabolism" -- the maximum rate of heat production that the lamb can summon.

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© 1981 Elsevier Scientific Publishing Company

238

Clearly the total environmental effect experienced by a lamb will fluctuate from hour to hour and follows variations in air motion, temperature, radiative effects and the intensity and duration of rainfall, as has been demonstrated, for example, by Slee (1977) and McArthur and Monteith (1980) in measurements of the (electrical) energy required to maintain the temperature of a simulated animal, by Alexander (1964) in climate-chamber research, and by Sykes et al. (1976) in field observations. Such investigations imply that almost all newborn lambs are subject to cold-stress (and a high metabolic demand) which is c o m p o u n d e d by low body (energy) reserves with which to meet the demand. An erosion of energy reserves from an already undernourished or sick lamb, particularly in breeds with short birthcoats, is recognised as contributing to the incidence of death in young lambs. In severe climates associated for example with hill farms, cold-stress might be the primary cause of death in newborn lambs. Such weakling stock might be at risk even in apparently moderate weather conditions such as those generally encountered in South-East England; it is this hypothesis that is investigated in this paper. MATERIALS AND METHODS

The fqrm Bradfield Farm lies mainly on a south-facing, undulating slope of the Berkshire Downs (some 17.5 km west of Reading) and leading down to the River Pang (Fig. 1). Of the 250 ha a large part is under cereals, about 40 ha are leys, and 16 ha are permanent pasture.

The flock The sheep flock in 1976 consisted of 400 breeding ewes of 4 different types (mainly Cadzows and Suffolk Crosses). In 1977 the flock size was increased to 486 by a batch of Scott half-bred gimmers, in 1978 further increased to 550, and in 1979 to 567. All lamb casualties were submitted for autopsy, careful records being kept of date of death, age and ewe of origin. Carcases classified as chilling or starvation in the field showed general congestion of the viscera, especially the lungs, and in most cases the stomachs were e m p t y or contained only a little milk. Table I presents the percentages of total field and house lamb losses due to chilling or starvation in the field over the years of the study.

Equipment In field investigations designed to evaluate the effect of meteorological factors on livestock, long-term interdisciplinary studies are c o m m o n l y desirable. Equally, an agriculturalist will c o m m o n l y require a swift resolution of a

239

Fig. 1. Sketch map of Bradfield Farm. Instrument sites are lettered A, E, G, H, J, K, L, M, N, P, Q, R, S, T -- H being the reference site, denoted y---n, and comprising Stevenson Screen with thermograph, air and grass-minimum thermometers, recording and digital cup-counters, recording and 5-in. rain-gauge. At other sites (symbol 0) are "air" minimum thermometers and cup-counter anemometers. Four contours are shown: 60, 75, 90 and 105 m ( ~ 2 0 0 - - 3 5 0 feet). TABLE I Analysis of lamb losses over the period of study, 1976--79

Ewes Total lambs born Total field and house losses Chilling/starvation losses in the field

1976

1977

1978

1979

400 745 85 6(7%)

486 943 116 20(17%)

550 1150 209 36(17%)

567 1133 238 77(32%)

production problem which is having, or could have, important economic consequences. Logistically, too, there are problems in supervising and maintaining site-monitoring equipment and in achieving adequate data-recovery regularly over an extended period (Smith, 1970}. Changes from year to year in gazing facilities, husbandry techniques, etc., add further dimensions of

240 difficulty. However, it was Gloyne's (1963) conviction that, given a sufficiency of frequent data, it should be possible to infer the basic features of at least the wind regime at any site after periods of study reckoned in weeks rather than years; this philosophy was adopted in the Bradfield exposure assessment. "Run-of-wind" measuring equipment (cup anemometers) exposed at 2 m height and minimum thermometers at I m were established near feed and watering areas and near significant hollows. Surveys with hand-held anemometers helped to establish a preliminary view of the association between local land form and local wind speed and direction. A chain of minimum thermometers was also established to monitor conditions within, and in the vicinity of, the ewe housing. Data were related to a reference site established in Annis field (Fig. 1) which housed a rain-gauge, a run-of-wind anemometer, a grass-minimum thermometer, maximum and minimum thermometers and a thermograph housed in a Stevenson screen at lamb height, and a minimum thermometer, screened simply and exposed at 1 m .as in the other fields. During the critical neonatal period daily readings were made. Subsequently, in 1978 and 1979, continuous meteorological recordings were made at lamb height using automatic equipment. The representativeness of such fixedpoint data in the present context may be open to question. The data can only provide an approximation to the actual environment experienced by a lamb capable of seeking shelter and within a protective flock. However, such data, even if not correct on an absolute scale, still provide a means of estimating days of relatively high or low environmental stress.

Heat loss from lambs Data obtained by Alexander (1964) and Smith (1973) provided the basis for expressing the environmental variables in terms of heat loss elicited from the young lamb. From Fig. 1 of Alexander (1964) it may be deduced, for example, that the heat loss (/1~) from a young, dry, 2-kg lamb with a fine coat in still air at --32°C (/~ = 186 kcal m -2 h -I ) is the same as that from the same lamb with a dry coat in a 5.5 m s -1 (12 m.p.h.) wind and at an air temperature of --4°C, or with a wet coat in a 5.5 m s -1 wind when the air temperature is + 13°C. The influences of wind and particularly a wet birthcoat are clear. Corrections to the environmental temperature Ta can be made to allow for a "radiative environment" most apparent under clear-sky conditions. Smith (1973) suggests decreasing Ta by up to 5°C depending on cloud-cover, and such corrections were applied before estimating/~. Although Alexander's work was on young merino lambs, the results were taken as applicable to the Bradfield lamb flock, a not unreasonable assumption according to Alexander (pers. comm., 1978). In 1978 the environmental heat demand of the environment over fourhourly periods was estimated as multiples of the basal metabolic rate M s , from the observed combination of wind, temperature and rainfall (it being

241

assumed that rainfall wetted the coat instantly and that the cessation of rain similarly marked the return of the birthcoat to the dry state). The heat loss estimates were accumulated for one, two and three days prior to lamb death and tabulated against the total numbers of lamb field deaths occurring within equal increments of the cumulative energy demand/l~. If the observed deaths are n o t weather-related, every day would hold an equal probability for the death of lambs. M was therefore calculated for every day and deaths allocated in proportion to the number of days with a given r a n g e o f energy demand to give an " e x p e c t e d " death distribution. Since the dead lambs were recovered only once daily in 1978, the time of death is in question; the assumption was made that all lamb deaths occurred overnight. In 1979 hourly estimates of the heat demands were made -- and lambs were recovered twice daily (confirming that deaths indeed occurred predominantly overnight). WEATHER CONDITIONSDURINGLAMBING

Bradfield weather for the four years is now briefly described. 1976

After a mild, dry winter, the spring and summer of 1976 continued to give exceptionally low rainfall. Less than 50% of average rainfall fell in March and May; April and June had only 25% of average. At Bradfield there were only 14 days with rain between mid-March and the end of May, and only 8 had more than 0.5 mm. Strong westerly winds set in for two weeks at the end of March, followed, in the second half of April, by a spell of northerly winds. Temperatures were somewhat below average in March and April, but well above average in May and June. 1977

Air and earth temperatures were well above average from February until mid-March. A cold spell then set in until early April (encouraging extended stays in the housing for lambs born then). Late April produced strong, wet, westerly winds which slackened off during the first half of May. A dry northerly wind became established until early June. In the mid-March to end of May period there were 43 days with rain at Bradfield, of which 37 had rainfall of more than 0.5 mm. 1978

The mild spell at the end of February continued into the first week of March, under the influence of fresh to strong south-easterly winds. In mid-March, after a wet night on the 15th/16th, the temperature dropped sharply in a cold north-westerly airstream, and a belt of snow affected the

242

area, followed by wintry showers until the 17th. North-westerly gales occurred on the 20th and winds continued fresh to strong until the 29th.

Summary for March Windy and unsettled; temperatures mostly above normal but wintry at times. The first four days of April were dull and misty with rain or drizzle. A sunny spell with north-easterly wind set in on the 4th; early on the 10th a very cold northerly airstream arrived after a cold front crossed the area and a wintry spell followed, with overnight frosts and moderate snow.

Summary for April This was the coldest since 1956; cold, north-easterly winds predominated. Mostly cold or very cold, with wintry conditions during the second week. May started wet and cool but the second half was dry, sunny and very warm.

1979 (for graphical summary, see Fig. 2) March A wet, westerly airstream affected most of the country for the first part of the month. It became colder from the 13th with snow or sleet showers or rain widespread for the rest of the m o n t h and strong winds at the end of the month. Mean temperatures were below average, rainfall (including water equivalent of melted snow) was twice average.

April It was generally unsettled and cold for the first 12 days with fog patches on three consecutive nights from the 3rd. Easter (13th to 17th) was the warmest since 1949. Thereafter the weather was again unsettled. Mean temperatures were below average, with rainfall nearly twice average.

May May was unsettled and cool with rain and sleet in the first week, becoming dry and warm by the 12th and very warm on the 14th and 15th. Then it continued unsettled.

Summary This was the coldest spring since 1962, the wettest since 1947 (and the second wettest this century). RESULTS 1976--1978

In 1976 only six field deaths accredited by the Veterinary Unit to "stress" occurred -- they were of twins in Rushall field, perhaps justifying the high

243 1979 Morch ~-

April

I0 15 20 25

0t

~E

5

May

I0 15 20 25

5

I0 15

5

R

n

~

n

25 20 15

o

IO

.I--

0 -5 ~

IOOi-

~~ 2~

80

2~

40

Fig. 2. L a m b field d e a t h s in Back Lane, 1979, s h o w i n g daily w i n d run, rainfall a n d h e a t d e m a n d . T e m p e r a t u r e trace d r a w n f r o m daily m a x i m u m a n d m i n i m u m t e m p e r a t u r e s .

exposure rating deduced for this field (see Discussion of pasture exposure studies below). In 1977 rain and wind-run were recorded only at 24-h intervals, and rainfall and winds over shorter periods allocated on the basis of continuous records from Benson (20 km to the north) -- these data qualitatively indicated the tendency for increased death incidence to be associated with days posing higher environmental stress. It is the detailed data for 1978 and 1979 which form the basis of this Results Section. In 1978 the mortality incidence was studied for the March period alone (when the majority of lambs dying were under 10 days old) and for the remaining period to early May {when age at death was in the range 20--60 days). A similar analysis was made in 1979. The Kolmogorov--Smirnov test (a test not limited by small samples) was used to test for significant differences between observed lamb field deaths (i.e. deaths not ascribed to organic disease) and the expected deaths in 1978 (the null hypothesis tested was that there was no difference between the two distributions). The difference between observed and expected mortality in 1978 in lambs less than 10 days old is significant at the

244

5% level when stress is accumulated over one day; there is no significant difference between the distributions either when stress is accumulated over several days, or as the lambs get older. DISCUSSION

Table II summarises information on 1978 "field losses" gained by the Veterinary Investigation Unit, Ministry of Agriculture, Fisheries and Food, Reading. Noteworthy features of the table are: (i) field deaths are dominated by twins and triplets; (ii) negative (or only small positive) weight gains are generally recorded in lamb deaths where the animal is under 10 days old; (iii) mortality is not confined predominantly to any one field. It is of interest to study the energy considerations for lamb survival. Alexander (1964) states that summit metabolism in young merinos is ~ 5 0 kcal h -1 (3-kg lamb) or ~ 2 0 0 kcal m -2 h -1 (i.e., about 4 times the basal metabolic rate MB ). In the present study, the maximum hourly energy demand was estimated to be 4.4 M s and the corresponding demands over 12 and 24 hours to be 49 and 94 MB respectively, environmental demands which, over the day, are close to summit demand and which exceed this demand over short periods. Total useful energy reserves in lambs from adequately fed ewes are ~ 2 5 9 k c a l k g -1 (i.e. 750kcal for a 3-kg lamb); the time to exhaust fat and glycogen reserves is hence of the order of 15h. Eales (1980) further points out that twins each weighing 3 kg have something like 30% more surface area through which heat is lost than a single 6-kg lamb; in addition, the twins have to compete for the available milk supply. The low, generally negative, liveweight gains in the young lambs (Table I) point to the energy intake being unable to match the energy lost to the environment; this continual erosion of energy, even if not fatal, is reflected in low liveweight gain and hence in the sale weight of the fat lamb. The Bradfield lambs were housed during the critical first day and overall were subjected to an altogether more moderate climate than that which can cause problems to upland flocks. Nevertheless, the 1978 Bradfield data indicate that there is a difference between observed and expected young lamb mortality. The implication is that young lambs (particularly twins or triplets), less than say 10 days old, can be at risk even in quite moderate weather. R E S U L T S 1979

In 1979 the severe March weather made the farm manager decide against putting the lambs out between the 14th and 22nd or the 24th and 26th March; 185 were put out in Back Lane on the 13th and a further 288 on the 23rd. Many lambs were, thus, between 2 and 10 days old when first exposed to climatic rigours. Details of the field losses, daily weather and heat demand are displayed in Fig. 2 and Table III. Three periods of particularly poor weather may be distinguished between mid-March and early April.

245 T A B L E II Field losses ; s u m m a r y of l o c a t i o n , b i r t h weights, etc., at Bradfield, 1978 Date o f retrieval

Age (days)

Single (S), T w i n (W) or Triplet (T)

Birth weight (kg)

Weight gain (kg)

Field*

M a r c h 14

2 2 4 1 5 4 4 2 7 2 5 4 7 8 7 2 18 9 4 22 15 7 7 6 6 18

W T T W W W T W T T W W T W W W W W W T T T T S W

4.76 2.72 3.63 3.18 5.67 3.63 3.97 4.65 3.29 3.63 3.86 5.44 4.31 3.86 4.88 4.31 4.08 3.06 3.29 4.08 2.60 3.18 2.95 2.60 3.40

+0.34 --0.22 --0.13 --0.18 --0.47 --0.18 +0.58 --0.12 +0.41 --0.03 --0.08 --0.57 --0.16 --0.11 --1.03 --0.07

A B D B O A A B D D B B D B B A B B B A D D D O A O

10 6 6 26 21 15 14 3 3 -17 15 14 6 22 14 31 24 11

T W W T W T W W W -W T T S W T W W W

2.72 3.18 3.40 3.06 3.63 3.40 3.40 5.44 2.95

+0.39 +0.02 +2.18 +3.27 +0.24 +0.55 +0.51 --0.05

3.18 2.83 3.29 3.52 4.54 2.95 3.52 5.22 4.08

--0.18 --0.83 --0.04 --0.32 +0.16 +0.33 +1.22 +0.11 --O.08

15 16

18 19 23 25

27

29

April

2

6 7

10

14 15

--0.12 +2.08 +0.64 --0.01 --0.05 +0.07 +0.24

D A B D B D B A B B B A D O A D B B B

246 TABLE II Date of retrieval

Age (days)

Single (S), Twin (W) or Triplet (T)

Birth weight (kg)

Weight gain (kg)

Field*

32 20 27 25 19 37 36 38 36 46 49

T W T T T W W W T S T

3.40 4.65 2.49 3.06 3.86 3.29 4.54 3.40 6.00 5.56 6.75

+1.90 + 4 .5 5 +1.80 + 2 .6 5 + 0 .3 9 + 6 .4 1 + 6 .9 6 +0.60 --1.28 + 4 .8 7 --0.67

D A D D D B B D D O D

2

28 35 39

T T T

4.55 3.62 2.72

+1.12 +1.14 +5.90

D D D

8

61 47 52 34 46 66 . 71

T T S W T T

4.08 2.95 4.76 4.08 3.86 2.72

+4.12 + 2 .1 5 +12.24 +1.92 +0.49 + 4 .2 3

D D O B D D

+3.06

D

16 19

21

27 28 May

(continued)

15 16 20

.

. T

.

. 4.42

*A, Annis field, twins (Mules + Cadzow); B, Back Lane, twins (Scots Cross + 1/2 bred); D, Bayleys Dene, triplets; O, Owl Pit, singles.

Mortality amongst young lambs was expressed as a percentage of those o u t on pasture and under 10 days old. Thus, for example, the 14 deaths on 13th/14th March represented 7.65% mortality, and the 6 deaths on 29th/ 30th March 4% mortality. The significant difference between the observed and expected percentage mortality in y o u n g lambs within three equal daily heat-demand categories (low, medium and high) is displayed in Table IV(a). Table IV(b), the corresponding table for older lambs dying after 7th April, shows no such significant difference, reflecting the 1978 findings. Similarly, high significance was displayed between percentage deaths and heat demand when summed over 12 h but was n o t apparent for heat demand summed over 48h. Peak hourly and dally heat demands for example for 13th/14th, 16th, and 28th/29th March were in excess of the summit metabolic rates for the corresponding periods and were in excess of 1978 values as well as persisting longer (for more than 36 h on some occasions).

III

* To 0 8 . 0 0 hours.

deaths n u m b e r on pasture 12-h h e a t demand ( × M s ) 24-h h e a t demand ( x M B )

"Pick-up "date

deaths n u m b e r on pasture 12-h heat d e m a n d ( x M B ) 24-h heat d e m a n d ( x M B )

"Pick-up" date

deaths n u m b e r on pasture 12-h heat demand* ( x M B ) 24-h heat demand ( × M B )

"Pick -up" date

1142 31 64

25

April

1 953 29 56

1

April

8

March

30 66

3

26

2 961 32 65

2

44 76

9

1154 28 57

27

1 972 30 61

3

34 72

10

32 59

28

992 32 59

4

35 72

11

29 56

6

29

999 31 60

5

35 66

12

31 59

30

2 1004 33 63

6

2 185 29 62

13

1 31 59

1

May

1018 28 59

7

46 94

14

14

2 33 77

2

4 1018 38 81

8

37 85

15

33 68

3

1 1023 38 68

9

37 78

2

16

32 64

4

3 1044 26 52

10

41 89

17

Lambs on pasture, field losses and corresponding 12- and 24-h heat demands, Back Lane, 1979

TABLE

30 58

1

5

1064 32 58

11

32 65

18

30 60

6

1082 31 61

12

33 64

1

19

2 29 55

7

31 63

13

32 65

20

28 56

8

1097 26 49

14

36 67

1

21

28 51

9

27 47

3

15

39 78

22

27 50

10

2 1110 27 45

16

1 477 35 68

23

37 72

11

31 55

1

17

35 63

1

24

25 53

12

1 1120 32 60

18

26

24 44

13

28 51

1

19

2 ~653 39 40 71 78

2

25

1 1123 32 59

20

765 40 80

27

31 57

21

5 829 42 83

28

30 57

22

4 874 50 89

29

1134 31 63

23

6 914 42 85

30

31 70

24

934 31 61

31

t~

248 TABLE IV Expected (e) and observed (o) lamb mortality (expressed as a percentage of those on pasture and under 10 days old) as a function of environmental heat demand during the 24 h preceding death (a) 8 March--6 April* (b) 7 April--14 May* Mortality

e o

Heat demand (×MB) 55--68

69--81

82--94

15 8

8 3.5

6 17.5

* P ~-- 0.001.

Mortality

e o

Heat demand (XMB) 43--55

56--68

69--81

6.3 6.9

16.8 14.4

2.8 4.6

* No significant difference between distributions.

DISCUSSION Some 60% o f lamb deaths were attributable to days when the heat demand over 24 h exceeded 80 MB ; the corresponding demand above which 75% of deaths occurred was 70 MB. Taking the total mortality in day-old lambs over the eight 24-h periods with M ~ 80 MB, the mortality of light, medium and heavy lambs is f ound to be 14.5, 5.2 and nil (expressed as percentages of their respective populations of 48, 171 and 35). On one night, 1 3 t h / 1 4 t h March, 6 of the 14 (i.e. 43%) light lambs ( ~ 3 kg) died; only 6 o u t of 51 medium lambs (3--5 kg) died. None of the 4 heaviest ( ~ 5 kg) succumbed. The practical implications of the critical 24-h heat demand of 80 MB mentioned for y o u n g lambs may, by reference to Alexander's (1964) Fig. 1, be summarised as follows. Wet lambs may be in danger in winds of ca. 10 m.p.h, even at a temperature o f ca.÷2 0 °C; the corresponding " d a n g e r " t em perat ure for dry lambs under these windy conditions is 0°C. Even a wind o f below 5 m.p.h, can cause problems to wet lambs when the t em perat ure is below 10°C (clear skies accentuate the danger). The t em pe r at ure must be below --10°C if dry lambs are to be at risk in tranquil conditions. (Calculations show t hat this 24-h heat d eman d of 80 MB was attained on at least 15 occasions in Northern England and the Borders in March and April 1979.) FORECASTING THE HEAT LOSS FACTOR ("WIND-CHILL") In the springs of 1980 and 1981, trial daily forecasts of likely hazardous weather for y o u n g lambs were prepared for the Bradfield area with the cooperation of the Meteorological Offices at R.A.F. Benson (Oxon) and Upavon (Wilts). Briefly, the 2 4 h beginning 08.00 local time were divided into 6 four-hourly periods, for which temperatures and wind speed (reduced to lamb-height in the Bradfield area) and the occurrence or n o t of rain were

249

forecast. Wind-chill indices were totalled (Table V (a) and (b)) and passed, by telephone, to the farm manager, whose initiative it was to make contact. Should a "warning" level be attained ( M ~ 8 0 ) the decision might be, for example, to keep young lambs housed for a further day or so. An example of the forecast (and actual) wind-chill for 27th March 1980 is shown in Table V(c) (Starr, 1980). The forecasts were followed meticulously in 1981; the farm manager reported only 1% field losses.

TABLE V Wind-chill indices for w e t c o a t a n d d r y c o a t (a) Wet coat indices Wind s p e e d at Bradfield ( k n o t s )

~20 11--20 ~<10

(b) Dry coat indices

T e m p e r a t u r e (°C) ~5

5 to 0

0 to - - 5

~--5

:>0

0 to --5

~--5

17 16 14

18 17 15

19 18 16

20 19 17

12 11 10

13 12 11

15 13 12

(c) F o r e c a s t a n d a c t u a l wind-chill o n 2 7 t h M a r c h 1 9 8 0 for Bradfield F a r m Factor

Period (local t i m e ) 08.00 to 12.00

12.00 to 16.00

16.00 to 20.00

20.00 to 24.00

00.01 to 04.00

04.00 to 08.00

10 12

10 11

10 9

7 7

7 6

7 7

15 15

12 15

10 12

10 8

10 4

10 10

yes yes

no no

yes no

yes no

yes yes

yes yes

16 16

11 16

14 11

14 10

14 14

14 14

Total

Temp. (° C) fore cast actual

Windspeed (knots) forecast actual

Rain forecast actual

Wind-chill * forecast actual

* A d d t h e six scores t o give the f o r e c a s t 24-h heat-loss f a c t o r M. I f M ~ 9 5 m a y b e c r i t i c a l ; / ~ ~ 9 0 - - 9 4 , d a n g e r ; M - - 8 0 - - 8 9 , w a r n i n g ; M---- ~ 80, green.

83 81 conditions

DISCUSSION OF PASTURE EXPOSURE STUDIES

The wind exposure table (exposures relative to Annis field) is presented in Table VI; the influence of topography and tree configuration can be clearly

SW--W W--NW NW--N

S--SW

N--NE NE--E E--SE SE--S

Direction

76 83 114 116 149 170 120 82

E

A

76 86 114 121 111 100 73 72

Pit

Owl

68 70 69 120 113 95 61 58

Bayleys Dene G 100 100 100 100 100 100 100 100

H --71 68 83 86 ---

J

A n n i s Field

49 53 35 49 70 91 104 49

K 89 89 59 61 52 55 71 87

L 73 74 89 178 206 175 108 103

M 69 -83 160 164 142 81 83

N

Rushall

82 91 104 127 151 147 97 91

P 82 -64 90 72 79 50 60

Q 72 80 74 124 113 93 58 71

R 61 66 55 75 100 65 ---

S

Annis Cross

140 170 90 90 150 170 76 82

T

Back Lane

" R u n - o f - w i n d " at t h e sites i l l u s t r a t e d in Fig. 1 as a f u n c t i o n o f d i r e c t i o n a n d e x p r e s s e d as a p e r c e n t a g e o f t h e c o r r e s p o n d i n g " r u n " a t t h e r e f e r e n c e site H

T A B L E VI

O

bO

251

seen. For example: a westerly flow is channelled into the low-lying, treelined hollow site (K) in Annis field; under a westerly flow, wind speeds at site K are some 20% more than at the reference site H 16 m higher. The north/south-oriented hollow in Rushall field channelled a southerly flow to site R; the highly exposed nature of the feed area in Rushall Site (M) is apparent (winds from the south-west up to 200% of those at the reference site) as is the lack of shelter from the north-east and south-east in Back Lane. Although Annis Cross 2 site J is situated on a ridge and some 30 m higher than the Rushall feed area, it presents a comparatively sheltered area for most wind directions. Influences of various features of the landscape on airflow have been remarked by Gloyne (1976), Hogg (1964/65) and others. Minimum-temperature data showed the marked severity of frost hollows in Rushall and Bayleys Dene compared with other sites (both generally 2--3°C colder on calm, frosty (radiation) nights). The available data suggest, therefore, that Rushall field and Back Lane present a considerable exposure hazard for livestock and that the potentially weakest lambs should be placed, for example, in Annis Cross 2 (Starr, 1978). The enforced use of shelter by ewes and lambs has been noted (Egan et al., 1972; Lynch et al., 1980) to be a major contributory factor in reducing neonatal lamb deaths; indeed, Cresswell and Thomson (1964) found that even the small amount of shelter from wind afforded by " t u s s o c k y " pasture could be important for lambs, and Munro (1962) made qualitative estimates of the efficacy of shelter due to land-form from daily grazing patterns. The gradation of minimum temperatures on radiation nights illustrated that drainage of cold air occurred down the south-facing slope (of about 1 in 7) toward the ewe housing. Minimum temperatures within the housing were 1°C higher than outside, irrespective of the presence of the ewes. The data show, then, that substantial variations in wind speed (and frost potential) can occur even between one field and the next; higher ground is not necessarily more exposed, nor low-lying areas more sheltered. Above all, the survey demonstrates the potential of assessing the exposure of pastures. Agriculturalists could use such data in choosing feeding sites, in allocating weakling stock and, in longer term, in assessing the need for establishing shelter belts. ACKNOWLEDGEMENTS

I am most grateful for the help afforded me by Mr. D. Ostler now of the Veterinary Investigation (VI) Unit, M.A.F.F., Wolverhampton, who initiated my participation in his lamb mortality study, Mr. J. Bishop and his staff of Bradfield/Rushall Farm, and Mr. J. Baxter (M.A.F.F.) in taking meteorological readings at various times, Mr. H. Pettit (Meteorological Office, R.A.F. Benson) and the S. Met. O., R.A.F. Upavon, for cooperation with forecasts, Mr. J. Gloster (Meteorological Office) for his contributions to analysis of the 1979 data, and to.Mrs. Morris for her work in extracting data and Dr. Purvis and Mr. Kirby of the VI Unit Reading for many discussions and constructive comments and supplying the veterinary data.

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This paper is published by the kind permission of the Director-General, Meteorological Office, Bracknell, Berkshire. REFERENCES Alexander, G., 1964. Lamb survival:physiological considerations. Proc. Aust. Soc. Anim. Prod., 5: 113--122. Blaxter, K.L., 1962. The reactions of cattle and sheep to the stress of cold environments. Ministr. Agric. Fish. Food. Symp. on Shelter Research, Aberystwyth, 1962, pp. 31-39. Cresswell, E. and Thomson, W., 1964. An introductory study of air-flow and temperature at grazing-sheep heights. Emp. J. Exp. Agric., 32: 131--135. Eales, F., 1980. Determinants of heat production in new-born lambs. Int. J. Biometeorol., 24: 157--166. Egan, J.K., McLaughlin, J.W., Thompson, R.L. and McIntyre, J.S., 1972. The importance of shelter in reducing neo-natal lamb deaths. Aust. J. Exp. Agric. Anita. Husb., 12: 470--473. Gloyne, R.W., 1963. A contribution on methods of investigating the effects of weather and climate on farm livestock under normal management in the British Isles. In: S.W. Tromp and W.H. Weihe (Editors), Biometeorology, Vol. 2, Part 1. Pergamon, Oxford, pp. 389--394. Gloyne, R.W., 1976. Shelter in agriculture, forestry and horticulture. ADAS Q. Rev., 21 : 197--207. Hogg, W.H., 1964/65. Measurements of the shelter effect of landforms and other topographical features, and of artificial windbreaks. Sci. Hortic., 17: 20--30. Joyce, J.P., Blaxter, K.L. and Park, C., 1966. The effect of natural outdoor environments on the energy requirements of sheep. Res. Vet. Sci., 7: 342--359. Lynch, J.J., Mottershead, B.E. and Alexander, G., 1980. Sheltering behaviour and lamb mortality amongst shorn Merino ewes lambing in paddocks with a restricted area of shelter or no shelter. Appl. Anim. Ethol., 6: 163--174. McArthur, A. and Monteith, J.L., 1980. Air movement and heat loss from sheep. Proc. R. Soc. London, Ser. B, 209: 187--237. Munro, J., 1962. The use of natural shelter by hill sheep. Anim. Prod., 4: 343--349. Obst, J.M. and Ellis, J.V., 1977. Weather, ewe behaviour and lamb mortality. Agric. Rec., 4 (July). Purvis, G., Ostler, D., Start, J.R., Baxter, J., Bishop, J., James, A., Dunn, P. and McClintock, M., 1979. Lamb mortality in a commercial lowland sheep flock with reference to the influence of climate and economics. Vet. Rec., 104: 241--242. Scottish Agricultural Colleges, 1975. Perinatal losses in sheep. Report of a Symposium at Stirling University, Scotland, 5--6 February 1975. Slee, J., 1977. Anita. Breed. Res. Org. Rep., (1977) 11--16. Smith, C.V., 1970. Meteorological observations in animal experiments. World Meteorological Organisation, Tech. Note 122. Smith, C.V., 1973. Critical temperatures of young beef cattle. Unpublished Agricultural Memorandum 526. Copies available from the Library, Meteorological Office, London Road, Bracknell, Berks. Starr, J.R., 1978. Lamb mortality in a commercial lowland sheep flock. Unpublished Agricultural Memorandum 865. Copies available from the Library, Meteorological Office, London Road, Bracknell, Berks. Starr, J.R., 1980. Forecasting "wind-chill" in lambs -- Spring 1980. Unpublished Agricultural Memorandum 891. Copies available from the Library, Meteorological Office, London Road, Bracknell, Berks. Sykes A.R., Griffiths, R.G. and Slee, J., 1976. Influence of breed, birth weight and weather on the body temperature of new-born lambs. Anim. Prod., 22: 395--402.