Onderstepo01·t Journal of Veterinary Sc·ience and Animal Industry, Volmne 14, Numbers 1 and 2, J anum·y and April, 1940.
Printed in the Union of South Africa by the Government Printer, Pretoria.
11.-The Carotene Content of Some South African Feeds.
By S. J. MYBURGH, Section of Biochemistry, Onderstepoort.
IN a ]HeYious publication (1835), it \Yas stated, that a Vitamin-A deficiency probably exists under ranching condition. during the dry period of the year in South Africa. There are indications of such u deficiency clue to the low carotene content of some mature vel<l grasses, whid1 ''"ere analyzed. To test the existence of such a short- age, it \Yas eonsiclerecl important that analyses of grass samples hon1 the ,-elcl or unde1· vehl conrlition,;, be carried out.
Since the preservation and conservation of grass hays for the feeding of animal,; iu the dry periods in the sununer-ruinfall areas i,; not· a general practice, the cattle on ranehes have to seek thei1·
feed on the veld. Due, JHohabl,Y, to hig-h temperatures in the summer and eYen in autumn, pasture g-rasses become mature very soon and the l'esult i,.; a rapid decline in the e;notene r-ontPnt of f'H'n the he;;t nutritional ~trasses.
Hart and Guilbed 11 9a3) have fonnd that, under ranching con- ditions in California, cattle suffer from a Vitamin-A deficiency during the dry spells. ·when the dry seasons were unusually prolonged the result was not onl_v an actual shortag-e of carotene intake, but as a result poor g-rowth, failure in reproduction, abortio11S, weak and sickly offspring- amongst range cattle. Experimentally these faets were established by them in their classical ''"ork (1931) on cattle, swine and sheep, on lo"· levels of Vitamin-A in rations. ~Iinimum clail)' requirements ''"ere thus found and given as 26-30 micrograms carotene per Kilog-ram body weight. Excellent growth could still be obtainerl at the.-e low levels of carotene (or Vitamin A), intake, hut the storage over a long period was small.
An excellent study can thus he earried out in conjunction with a g-eneral suney o:f South African veld grasses, and valuable infor- mation so ohtainecl, concerning- the carotene content of our well- known g-rasses as an indication of a deficienc)· that may or ma:v 11ot PXist.
The years 1932 and 1933 were exceedingly dry periorls with no rain for months to bring· ahout the onset of new growth of gTass. During .JHne . . July, and Aug'ust, 1932, there wns no rai.n nt all,
421
TilE CAROTENE CONTEXT OJ' SOUTH AFltiCAN FEEDS.
and again May (June, 0·23 inches), July, August, September, and October, 1930, were dry months. Unluckily this work was begun only in 1935 and thus ct very important period was missed. It is hoped that "·ith the occurrence o£ such a period again, the necessary ir1fornwtion will be obtained.
To obtain, as preliurinary work, an indication of the carotene l:ontent of some important and indigenous g-rasses, the writer decided to cut from esta blishecl experimenta 1 plots, moll thly samples and analyse them. These gTasses when once established, were left to grow under naturd chmat.ie conditions.
The plots 1·eferred to here are described in detail by J. G.
LomY (1 938) nn rl the grasses selected for monthly cuttings over the period of a year wel'f• : Chloris gay ana, Diqita1'1:a en'antha variety Stolonifero., Cenchrvs C'l"l-im·is, Rragrostis plana, Pawic11111 mrr.1·i 11111111
ilnd Setaria lindenbPr.r;iana.
Fu rthenno1·e, a second serie,; of analyses ea roried out on sample::;
of g-rasses cut on experimental plots at Onderstepoort, at periods of one month, two months, three months and four months. The grasses in question were five in all, namely Chloris .r;ayana, Seta?·'ia hnden-
!Jero?:nana, C enchrvs c·ilim·is, Digitaria pentz·i,: Pretoria Small, an rl l)rm·icum ma:.cimu1n. 1'he nutting of these gTasses was really intended to obtain in:fonnation on " 'l'he I nfluen<'e o:f Frequency of f'ntting on the Yield of Dry ~1atrer " (see .T. G. Louw, Hl!1R).
These gru,;:;es, therefon>, represent Ya rio us growth stages up to four months :for the season January i.o April (inclusive), 1936.
Lastly, air dried smnples of gTasses, cut on. the veld at monthly inter- vals and sent in from the experimental farms Armoedsvlakte, Vryburg (Bechnanaland) and Nooitgedacht, (hig·hveld, Transvaal)
"·ere analysed. These sample,; wen• cut for ann lysis during· the periorl August, 1935, to August, 193G, and anal.Ysed shortly after cutting. It is hoped to publish values :for the carotene content o:f veldgrass samples :for these :-treas for furthp1· periods up tn rlnte in the 11ear future.
ExPERL\lEXT.\L.
Samples for analyses were cut from selected experimental plots
:llld a central position was chosen for cutting. Care was bken, that samples were not cut at the same spots where previous monthly ruttings had been made. In other 'vorcls, a new position on ench plot was select,•<l :for earh month))' cutting-. Thns the effect of cutting un tlre carotene rontent was eliminated. 1'he fresh gTass ,..-as imme- diately bottled in closed jars ancl analysed soon afterwards in the laboratory. )Ioisture cletermina tions were made on the :fresh grass b,v weighing in closed jars, before and after drying at 103° C. Fresh
QTet~n grasses could be analysed without milling or mry pretreatment, Pxcept that of cutting into small hits. Old, dry and mature gTasses were firstly milled and sampleil for analyses. All values for caro- r.ene are given. in the tables, as ll'ell as moisture values where nef'eH-
~ary, and meteorological data for the periods conrerned. 422
S. J. MYllURGH.
M ETHoD AND 'rEcH:\'IQl:E oF ANALYsEs voR CARoTENE.
The method employed is the same as that previously adopted by the author (1935) and advocated by Guilbert (1934), except for a minor modification. After the extraction and saponification of the fat soluble pigments with alcoholic potash, the writer re-extracted the alcoholic extracts with petroleum ether instead of with ethyl- ether. This simplified the procedure eonsiderably.
The standard for colorimetric comparison was the Dve-Standard of Guilbert, where 1 liil'e of tbe dye solution represent~ecl 2·4 mg.
carotene.
DrscvsSION.
As previom;ly indicated by the writer (1935), there is a decrease in camtene for pasture grass samples in the veld or for established gTasses on plots during the dry months (winter) and this is especially marked in late "·inter. This fact can be clearly seen by studying '!'able VII and Table VIII, where values are given for veld grasses
£rom the experimental farms at Vryburg and Ennelo. Rather dry winters were experienced during the years 193!) ancl ] 936 as can be gathered from Table IX.
Calculating the :werage chi].'· i11take of caTotene for cattle on the basis given by Guilbert (1937), namely :!5 micrograms per Kilogram l.Jocly ·weigl1 t and furthermore, assuming that an 800 lbs. buvine consumed 011 an <n-erage 15 lbs. <ln· "·eight of hay daily, \Ye find that a deficiency of carotene existed for the dry monthR of .June, .July, August, and September, (1935 and 19JG). 'l'his lleficiency is based on the formula. of Guilbert according to whieh the animal requires 9 millig-ram carotene per day to prevent the onset of clinical llymptoms. Substantially more carotr.ne would be necessary to build up a resene and this the animal is able to do during the summer months. On the other hand depletion of carotene takes place very rapidly. Guilbert (1935) gives the daily withdrawals from storage as from 9 to 11 microgram~ per Kilogram body weight. In compari- son with 1935 an<l 19=-lG on the other hand, the vears 1938 and 1939 were good years "·ith substantial falls of rain eYen as late as ~Iay and July, 1939. Accordingly, the YalueR for the season August, 1938, to August, 1939, for which reference should be made to '!'able I or I (a), were Pxceptionally high, uncl taken on an :n-erage there
exi~ted no de.ficieney for the six grasses analysed. Some of the individual grasses did sho,\· a markedly serious fall in c:notene yerging on the c1·itical value to prevent clinical symptom .. , iu the late \\·inter of 1938. 'l'his was largely due to a fairly dry spell from May until September \\·hen only small aYerage rainfalls took place. Some types of grasse", as for instance Chloris gayana will
;;prout in "·inter even with little rain whereas Panictvrn ma:c·imu,m nuely does, and thus ne"l'l· gro>d,h is accompanied by an inrrease in Carotene. It is anticipated that, during the seasons "·hen the rainfall i,; low and the hot weather hastpns the maturing of the Yeld grasses, a prolonged dry spell starting with winter would result in a. likely shortage of Vitamin A, if not in a serious deficiene.v in the summer- rainfall areas.
423
TA11LE I. Carotene Content of Ura8.ses cut monthly front E;tperimenlal Plots (Undwrstepoort).
..,..
'l'ype of Grass. 1. Chloris gayaua ("Rhode~ Grass") 2. Digita,-ia e1·iantha var. Stolo· nifera ~ 3. Cenchrus cilian:s (" Bnffcls· gra.s ") 4. E·ragrostis plano ......... . ii. Panicum maximum (Hairy "Buffelsgras ") 6. Setarin lindenbergiana ..... .
Veriod. Carotene ~[oisture Ca.rotcne Moisture Carotene Moisture Carotene Moisture Carotene Moisture Carotene l\Ioisture
E:vJJtessed as 111 (1. 71er cent. on N at1lml Hasis. Aug.
[ {'g~;~· . I
Sept.I
Oct. 1 Xov.I
Dec. jjT 0 ~~1. \
Veb. [ l\[arciLI April. l\[a.y.I hn f'. l
July.I
----'---- 0·441 ': ::
8·30 0·05 21·40 I 0·05 5·00 0·092·62 :lO·OO 1·31 :?3·80 0·13 38·00 0-:W 14·20 0·21
2·47 3:1·30 4·24 32·00 4·29 43·10 0·:20 14.·00 l·13 14. 80 I 26.00 I 12.30 o · 4 7 o · 40 I 1 · 93 15. 20
I
31 . 10 I 9. 504·00 68·50 5·82 63·50 5·33 n·80 0·80 30·;)0
1 4::~: I
4·34 71·60 10·90 72·20 7·35 82·20 8·00 45·10 7·70 7.5·00 2·91
I
9·41 16·00 72·10----~--- 2·55 1 3. 84 3·36 1·16 I 1·74 66·00 I 68·30 I 68·60 I 66·60 3·95 I 2·80 I 2·77 I 0·79 65. 40 68. 10 1 55. 5o 3o. oo 2·74 4·57 1·28 0·14 56. 30 72. 50 69. 80 30. 70 5·i)7 7·74. 3·15 0·10 I 1
41· ~0 170.30 !i4.. ~0 20.00 l·iil 2·62 2·17 0·18 71·10 '72·i'>0 54·70 131·70 4.
5 7 1
3 . 60 3 . 20 1o .
34 60 . 20 I 60. 00 50 . 00I
34 . 50 --~ --·55·50 0·48 16·60 0·59 21·40 0·37 20·00 0·14 30·00 0·33 18·10
0·531 27·70 0·33 18·10 0·21 24·30 0·15 17·00 0·04 25·00 0·161 126·50
I
0·64 11·60 0·27 7·70 0·15 8·30 0·24 13·80 0·06 7·40 0·24 6·10
,.., ~ i:'J C':l > ~ 0 ,.., i:'J z i:'J 0 0
z
~z
,.., 0 ":j [/] 0 d ,.., ~ > ,.,. ~ >-< n > ~ >,j i:'J r.; lj [/J~ O•
'l'A BLE I (o). CorolenP ContPnt of Grasses cnt monthly from. l!J:vpwtimental Plots (OndPrsiPJHwrt). Rxp1'essed as mg. percentage on dry basis. Type of Geass.
I
Period.I t9~~ -- I
Sept.I
Oct.I ~ov ~l De c ~ I i~~~-
r Feb.I
)fa.rch., April./M~t y. I
Junc.,.July.I
Aug. 1. Chlo1·is gayana ("Rhodes-
0·;)2 1
:~. 75 3·70 12· 70 115· 30 -7·50 12·10 10·70 3·47 :~·91 0·73 0·71 Grass") 2. Digitaria e1·iantha var. Stolo--0·13 1·72 6·23 16·00 39·20 -11·40 8·94 6·23 1·13 0·58 0·40 0·29 nifera 3. Cenchrus ciliaris (" Buft.ds--0·06 0·21 7·54 20·30 41·30 -6·26 16·60 4·23 0·20 0·75 0·28 0·16 gra.s ") i. FJragrostis plana ........ -0·05 0·23 0·23 1·15 14·70 -9·56 26·10 6·92 0·12 0·46 0·18 0·28 5. Panicum max·;·mum (Hairy -O·ll 0·28 1·29 3·91 :~0-85 -5·98 9·52 6·10 0·23 0·20 0·05 0·06 " Buffelsgras ") 6. Setaria lindenbe1·giana .... _ . -0·55 0·58 2·13 3·46 :33· 70 -ll·50 9·00 6·40 0·52 0·40 0·22 0·25 ------- TABLE 1I. Uod.IJ lntalce of Co·rotene ( ~~tg.) for Cattle (800 lb. 1/i·ue-weight) tJalcu.lotcrl front Table I (a). Type or Grass.I
Aug.I I
___ ·. 1938. Sept. Oct.[Nov. , De c . / l9~ 3 ~·- 1
Feb.,.\-Iarch.,April.l May.I ~ run e. ,
July., Aug. J. Chlor~·s ga.J;nna (" !l.hodt>s Grass'.). ............ . 2. Digitm·ia e1·ianlha var. Stolon·~{ em ............. . 3. C'enclms ciliaris (" BnffPisgras '') ............... . 4. Mmgrostis ?'la.na ...................... . 5. Panicwn ma~·imwn (Hair_,. "]3utl'elsgras ") ....... . H. Setm·ia lindenbe1·g£cma ...........••..35
I
253I
250 I 857 116 420 11,080 12 9 4 314 I 508 11,370 12 7
16 19
15 87
78 264 12
-506 818 -769 603 -423 1,120 -645 1,760 -403 642 -775 607 ---
723 234 264 49 49 420 76 39 27 20 285 14 51 19 11 467 8 31 12 19 414 15 13 4 4 433 35 27 15 17 --
--
------ "I ,,, -"'- ~~ - '~ ,. ~ · ~
9 9 91 91 9 9 9 9Average for o Grasses ... · ... · .. · J
~:
::~ I ~:: ::~ ~ ~
=}=[i=n=im=u=,n=Da~~~equir<'m<'nt according to Guilbert .. 1===;}1=;;-1-9 =;;-1=== I I ' --·~ ~ IS: ~ l:l:l c::l ;o Cl ~
THE CAROTEXE COKTENT OF SOUTH Al:'RICAK .FEEDS.
TABLE ITI.
J/ efeoTofog ical Datu- Onrle1·steJJOOI"t, Tra·ns·wa I.
Date.
1938.
,January ............... . February .................. .
~larch .................... . April.... . .............. , :May ........................ . ,Tune ......................... . July ............. , ......... . August .................... . September .................... . Oct-ober ................. . November .......... . DecembE'I'...... . ........ .
1939.
January ........... . February ........ , ............ . MarciL .................. . April. ..................•...
May ...................... . June ..................... . July ................ . August ...•.............
Average
l~ainfall.
(Tnches). 5·52 2·56 1·24 2·94 0·30 0·19 0 0·24 0·33 2· 81 1·23 10·70
5·24 8·82 2·7[) 0·07 2·54 0 2·44 0·40
Difference A \Tcrage
I
A veragc betweenMr,ximum .\1inimum .\llaximum Temperatru:c. Temperatu•·e. and Minimum
I
TempC'raturP.oF. 011.,, oF.
82·2 59·7 22·i5
79·4 58·8 20·6
86·9 52·2 34·'7
83·8 48·0 35·8
77·0 38·6 38·4-
68· I 35·6 32·5
60·5 34·3 26·2
66·6 36·6 30·0
74·9 39·8 35· I
82·5 55·4 27 ·I
86·7 52·6 34·1
84·4 54·:) 29·9
81· 8 m-4- 27·4-
84·0 .58·2 25·8
7i5· 7 48·6 27· I
74·2 44·2 30·0
74·0 39·1:1 34·1
68·5 32·8 35·7
65·4 33·8 31·6
70·2 37·5 32·7
- --- - -
'1' AJILJo: IV.
CaTotene Content of Plot Grasses cut from, Experimental Plots, Onderstepoort, expressed as mg. Per cent Carotene on Dry Bas£s.
Date of Analysi~.
1937.
J.:Monthly Uuttincs 6/1/36; 3/3/36;
2 3 4
4/2/36; i)/4/36 {pooled).
-li-Ionthly Cuttings 4/2/36; 6/4/36 (pooled)
-Monthly Cuttings 3/3/36 ...
·Monthly Cuttings 6/4/36 ...
I
Chlorisgayana.
I
2·36
2·00 0·90 0·85
I Setar·ia
I
liigitaria
linden· Cenchrus pentzii Panicum
I
ciliaris. Pretoria ma,x1:-rnu m,.
bergiana. Small.
I 3·00
I 2·40 1·86 :l-12
1·71 1·80 1·50 1·89
1·74 1·74 1·20
I
1·71
1·62 1·33 1·30 1·20
426
TAnLB V.
Daily Intake of Cm·otene (mg.) for Cattle (800 l/J. Live-weight) utlc1tloted from TrzlJlP l \'.
1-:\(onthly Cuttings .......... I
2-:\Ionthly Cuttings......
~-:\Ionthly Cuttings ......... . 1
'±-.\[onthly Cuttmgs ......... . AYcrage Minimnm Requirement. I
Chloris aayana.
159 135 61 i\7
!)
8etm·ia linden- be?·(Jiana.
202 l lii ll7 109 q
TABLE VI.
Cenchnts ciliaris
162 121
lhaitaTia pentzii l'retoria
Small.
l:?G 101 117 90 I 81 87 I
~--9-~-9-·.
l)an·icU'n~
1naxi1num ..
143 us
llii 81
!)
Meteorological JJata- Onderstepoort.
Date.
1935.
September ... .... .
...
. October ........ . . ...November. ....
.
... December ....... ...1936.
January ...
February. . . . . ... . ... March .... ...
..
. . . . ...
April. ....... ...
..
. . . ... . . May ..... .... . . .. ......
.. .. . .June ........•....... ...
July ......... ...
August .... ... . .
..
. . .. . . September ..................... October ............ ...November ... .... .. .. . . December ............ ...
Average Rainfall.
(Inches).
0·36 0·97 2·09 3·17
4·69 7·67 8·50 0·51 4·26 0 0 0 0·46 2·69 0·54 3·37
427
Difiercnee
Average Average between
1\Iaximum Minimum Maximum Temperature. Temperature. and Minimum
Temperature.
:F. F. F.
81 ·8 43·9 :n-o
91·1 fi2·2 :~8-9
88·2 55·5 32·7
87·6 -~6-8 30·8
87·6 58·1 29·5
85·2 58·3 26·9
80·0 56·1 23·9
79·8 49·0 30·8
71·4 40·6 30·8
72·1 33·4 38·7
70·7 33·6 37·1
76·0 36·2 39·8
77·6 40·6 37·0
83·2 51·4 31·8
82·2 54·5 27·7
88·0 55·ii 32·5
....
tv (/)· 'l'AnLE VII. A.-Carotene Content of Veld G1·ass-Annoeds'vlalcte, V7·ybur·g, ea;pressed as 'tnlf· pe1' cent. Carotene on Dry Bas·is. Aug.
I
Sept. Oct.I
Nov. Dec.I
Jttn. }"eb.j
March., April.I
~"Lay.I
June. July.I
Aug. 1935. 1936. ·06 ·10 I ·28 I ·28 I ·55 I 1·08 I 0·71 I 0·83 I 1·15 I 0·70 0·08 0·11 I 0·12 B.-Daily Intalce of Carotene (mg.) /m· Cattle (800 lb. Live-weight) calculated from Table VII.~~~- - I
Rept.I
Oct.I
Nov.I
Dec.I (
9a{6·. I
Feb.I
March.[ April.I
May.I
June.I
.July.I
Ang. 4 7I
19I
19 37 73 48I
56 77I
77I
5 7 8
I
II
I
I I
!) 9 9 9 9 9 9 9 9 9 !) I
Average minimum require- ment ... . 9 9 '1' AllLB VIII. A .-Carotene Conteut; of Veld G·ra:;s-E.Tpe1·imental Ji'arrn, Ennelo, ea:p1·essed ns mg. per cent. Carotene on Dry Bas·is. Aug. Sept.
I
Oct. Nov. Dec. Jan. Feb.I
March., April. May. June. July.I
Aug. 1935. I 1936. 0·18 I 0·08 0·13 I 0·36I
1·34I
1·00 I 1·07I
1·50 I 1·07 0·60 I 0·13 0·05 I 0·08 ·--- B.-Da·ily lntalce of Carotene (1ng.) jo1· Cattle (800 lb. 1-'i·ue-weight) calcttlatecl f1'07rL Table VIII.!
Aug. Sept.I
Oct.I
Nov.I
Dec.I
Jan. Feb.
I
March., April.I
May.I
June.I
July.I
Aug. I 1935· 1936.I
12 5 !) 24 !)0 fi7 72 101 72 40 9 3I
5 --- Average Minimum Itequire- ments ............ 9 !) 9 9 9 9 9 9 9 9 9 9I
9 ----~ ---------~ -------~~ t;j (") > ;>:!
~ z
t.>J (") 0z
..., t.>Jz
,.., 0 "! rfJ 0 ~~
> ~ i::J .... (') ~ "! t;j ~ rfJS. J. MYl:lUllGH.
Mete01'ological Data- A7'?noeds'vlakte, Vrybvrg, Bechuanaland.
Date.
193;3.
January ....... . February ................... . March ................... . April ..................... . May ..................... . June ....................... . J"uly ............. . August .................... . September .................. . October ................. . November .......... . December .......... .
1936.
January .. ................. . February ..................... . March ................ . April. ........................ . May ........................ . J"une ................. .
J"uly .................... . . .
Augu~t .................... .
Average.
Rainfall.
(Inches).
0·93 2·69 2·66 1·94 0·60 0 0 0 0·25 0·52 1·15 3·98 2·51 3·6L 3·23 0·06 1·11 0·02 0 0
Difference Average Average between Maximum Minimum Maximum Temperature. Temperature. andMinimum
Temperature.
F.
92·74 86·84 80·47 75·49 69·31 64·93 70·20 68·76 77·82 90·48 86·76 88·43 b7·74 85·94 78·98 79·96 68·27 69·13 67·84 74·50
F.
;36·15 56·99 53·;34 46·51 37·37 29·20 31·20 31·09 39·98 53·76 54·98 59·89 57·36 58·02 56·08 46·61 39·82 32·65 33·15 34·70
F.
36·59 29·85 26·93 28·98 31·94 35·73 39·00 37·67 37·84 36·72 31·78 28·54 30·28 29·96 22·90 33·35 28·45 26·48 34·72 39·80
SuMMARY.
l. Carotene values are given for six well-known grasses, cut monthly :from established plots under natural climatic conditions at Onderstepoort :from August, 1938, to August, 1939.
2. If the :n-erag-e bovine (800 lb. bouy "·eig-ht) requires !:) milli- grams carotene per day, then there exists no actual Vitamin-A deficiency for the season in question.
3. Carotene values are given for grasses cut at one-monthly up to four-monthly periods during the growth stage.
4. 'rhe aYerag·e intakes of carotene from these g-rasses are well above the minimum requirements o:f stock.
G. Carotene values are given for veld g-rasses cut on the Yeld frc:m the areas Vrybnrg, Bechuanaland and Ermelo on the HighYeld of the Transvaal during the years 1935 and 1936, and analysed shortly afterwards.
G. Here for the period in question, there is a strong indicntion of a likely Vitnmin A deficiency during the dry months.
429
THE CAROTENE CONTENT OF SOUTH AFRICAN FEEDS.
Acr<NOWLEDGEUENTS.
The writer appreciates useful advice readily g1ven by Dr.
A. I. Malan.
E1moRs 1N PuJ3LICATION, 1935.
(a) 50 c.c. of the stock solution to 1,000 c.c. etc., should reail 5·0 c.c. of the stock solution to 1,000 c.c., etc. See page 479.
(b) Value of Dye Standard used=2·7 per 1,000 c.c. carotene and not per cent., as given in Tables A and B. See page 479.
J.ITERATURE.
ATKESON, F. vV. et a/ (A ugu~t, 1937). Observations on the carotene content.
of some typical pasture plnnts. J. D(ci1·y Sc .. Vol. 20, :t\o. 8.
GUILBERT, H. R., A:'>D HART. G. H. (1934). Storage of vitamin A in cattlP.
J. of Nut1·it-ion, Vol. 8. Xo. 25.
GUILBERT, H. R., :\IlLLEH, R. F., Al':D HUGHES, E. H. (1937). The mini- mum vitamin A and carotene requirement of cattle, sheep and swine.
J. of Ntdrition, \T ol. 13, pp. 543-564.
GUILBERT, H. R. (1934). Determination of carotene as a means of estimating the vitamin A value of forage. lndns. and Eng. Ghem .. Vol. 6, p. 425.
HART, G. H., A:'>D GUILBERT, H. R. (1U3:1). Vitamin A defic)ency as related to reproduction in range cattle. Calif. Agric. Expt. Sta., Bulletin 560.
LOUW, J. G. (1938). The Influence of Frequency of Cutting on the Yield.
Chemical Composition, Digestibility and Kutritive Va I ue of some Grnss Species, Onderstepocn·t J. of Tlet. Sc. and A.nimal Indnsh·y, Vol. 11, No.1.
:\1.ACKINNEY, G. (1935). Properties of carotenes from certain roots and leaH•s at various stages of de,·elopment . .J. Biol. Ghem., Vol. 108, PIJ. '~5-49.
l\lEIGS, E. B., A:\"D CO:'{YERSR, H. T. (1936). \'itamin A requirements ol dairy cows for reproduction and lactation under practical conditions.
J. Doi1·y Sc., Vol. 19, p. 438.
r.IYBURGH, S. J. (1935). I. The carotene content of some South African feeds.
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