Interrelations of Milk-Fat, Milk-Protein and Milk-Energy Yield

Interrelations of Milk-Fat, Milk-Protein and Milk-Energy Yield

INTERRELATIONS OF MILK-FAT, M I L K - P R O T E I N AND MILK-ENERGY YIELD W. L. G A I N E S AND O. i~. O V E R M A N Illinois Agricultural Experiment...

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INTERRELATIONS OF MILK-FAT, M I L K - P R O T E I N AND MILK-ENERGY YIELD W. L. G A I N E S AND O. i~. O V E R M A N

Illinois Agricultural Experiment Station, Urbana, Illinois

This article grew out of an inquiry by the American Dairy Cattle Club. In the registration of dairy cattle the Club has taken the progressive step of requiring an estimate of milk-protein yield (in addition to the usual estiffiate of milk and milk-fat yield) of individual cows. The yield estimates are based on monthly tests covering the 3d-307th days of each lactation, or 305-day partial lactation system. A recent article by Dr. Goodale (1), geneticist for the Club, represents that protein is the most valuable of all the milk components and that it is desirable, by breeding, to increase the ratio of protein yield to yield of other milk solids (comparable to increasing the proportion of high-priced cuts in meat animals). 1 A breeding program along this line requires a practical field test for protein. Pending the development of such a test the question arises as to the possibility of estimating protein yield from the milk and fat yield. This question may be put in the form of the relation between fat percentage and protein percentage of the milk of individual cows. I f we know the 305-day fat percentage for a given cow, how accurately can we estimate her corresponding protein percentage ? The relation between fat and protein (as well as other milk components) for 3-day samples has been heretofore reported (2, 3). It is the purpose of this paper to apply the analyses of these 3-day samples to the appropriate milk yields to secure an estimate for individual cows for a partial lactation approximating the above 305-day period, and to present the interrelation between yields of various milk components, particularly milk fat, milk protein and milk energy. DATA AND ]YIETttODS

The chemical analyses (2, 3) were made on 3-day composite samples of the milk of individual cows in the University of Illinois herd. For the most Received f o r p u b l i c a t i o n J a n u a r y 8, 1938. 1 T h i s is a v e r y free v e r s i o n o f G o o d a l e ' s p a p e r , w h i c h in f a c t c o n t a i n s a m i x t u r e o f legal, economic a n d biologic p o i n t s , a n d u n f o r t u n a t e l y s o m e m i s i n f o r m a t i o n as to milk composition, l i e s t a t e s t h a t as we p a s s f r o m milk o f 3 p e r c e n t f a t to m i l k of 6 p e r cent f a t t h e p e r c e n t a g e o f lactose i n c r e a s e s f r o m 4.0 to 4.4 a n d t h e p e r c e n t a g e of a s h i n c r e a s e s f r o m .6 to .9. A s i d e f r o m t h e g r o s s errors involved in his a b s o l u t e lactose a n d a s h values it is quite c o n t r a r y to t h e well-known p r i n c i p l e of osmotic e q u i l i b r i u m i n m i l k secretion t h a t lactose p e r c e n t a g e a n d a s h p e r c e n t a g e s h o u l d v a r y so m a r k e d l y in t h e s a m e d i r e c t i o n : To m a i n t a i n a c o n s t a n t osmotic p r e s s u r e o f m i l k (equal to t h a t o f t h e blood) it is inevitable t h a t v a r i a t i o n s in lactose p e r c e n t a g e a n d a s h p e r c e n t a g e will t e n d to be i n opposite directions. 261

262

w.L. GAINES~ D O. R. OVERMAN

part the samples were taken at 5-week intervals, although there was some variation in this particular. The analysis of each sample included water, fat, protein (total nitrogen × 6.38), ash and lactose (lactose by difference). In all, 2426 samples were analyzed. Milk yield was determined by weighing each milking. In the present use of these data the analyses of the first 7 to 11 samples of the lactation have been applied each to an appropriate portion of the continuous milk-yield records (the sample being at approximately the center of the milk-yield portion) to estimate the total milk, fat, protein, lactose, ash and water yield for a period approximating the 3rd-307th days of lactation. The actual length of period represented varies somewhat but is here referred to as 305 days. As thus defined, 305-day partial lactations were available for 130 cows, represented by a total of 1519 samples (out of the 2426 referred to above). From these 305-day yields the average composition of the milk is derived and presented in Table 1, together with the identity of the cow, the number of samples, length of period represented and average milk yield per day for the period. In the table, 7 to 11 samples indicate a single partial lactation; !6 to 21 samples indicate two partial lactations combined as one; 27 to 32 samples indicate three partial lactations combined as one. Milk energy is estimated from the equation (4), E =93.12f+53.58p + 39.871 + 49.80a - .356w, in which E is calories of milk energy per kilogram of milk, f is fat percentage, p is protein percentage, 1 is lactose percentage, a is ash percentage, and w is water percentage. 2 RELATION BETWEEN FAT AND PROTEIN

The 305-day fat and protein percentage data of Table 1 are plotted in Figure 1. The correlation between the two is measured by the coefficient of correlation, r = .755. Protein is related to fat by the linear equation p = 2.10 + .346f) ± .085, shown by the straight line of Figure 1. From the standpoint of estimating an unknown 305-day protein percentage from a known 305-day fat percentage by the equation p = 2.10 + .346f applied individually in a large population of cows, the present data indicate that the estimate would be correct within .085 either plus or minus for oneSince, b y the method of analysis, f + p + 1 + a + w = 100, we have the algebraically equivalent equations : E =_+ 0 + 93.120f + 53.580p + 39.8701+ 4 9 . 8 0 0 a .356w = + 9312.0 + 0 f - 39.540p - 53.2501 - 4 3 . 3 2 0 a - 93.476w -- + 5358.0 + 39.540f + 0 p - 13.7101- 3 . 7 8 0 a - 53.936w =+3987.0+53.250f+13.710p_.+ 0 l+ 9.930a-40.226w = + 4980.0 + 43.320f + 3 . 7 8 0 p - 9.9301_+ 0 a - 50.156w =3~.6 + 93.476f + 53.936p + 40.2261 + 50.156a _+ 0 w A s a m a t t e r of convenience in c o m p u t a t i o n the l a s t one of these equations was used in e s t i m a t i n g E. I n a p p l y i n g the equation f, p, 1 a n d a were used to three decimals instead of the two reported in Table 1.

263

MILK-FAT~ MILK-PROTEIN AND ~ I L K - E N E R G Y YIELD TABLE

1

Composition of 305-day partial-lactation milk yields of 130 individual cows

Herd No.

Samples

Total period

Milk yield

Fat (f)

Prorein (p)

Laetose (1)

Ash

Water

(a)

(w)

Energy p e r kg. milk

No.

ds.

kg.

%

%

%

%

%

cal.

4.48 4.92 4.52 5.07 4.51 5.14 4.81 4.85 4.56 4.64 4.91 4.91 4.65 5.28

.67 .66 .69 .68 .70 .68 .69 .65 .67 .66 .67 .70 .71 .69

88.40 87.51 87.46 86.64 87.70 86.75 86.92 87.02 87.04 87.00 86.32 86.37 86.39 85.40

657 704 732 770 722 763 760 761 764 766 805 802 814 872

5.34 5.02 4.94 5.23 5.10 5.11 5.25 5.19 4.86 5.00 5.37 5.09 5.10 4.99 5.14 5.30 5.00

.71 .75 .71 .75 .69 .72 .69 .71 .70 .68 .69 .73 .73 .74 .73 .71 .78

87.24 87.35 87.73 86.91 87.29 87.26 87.39 87.09 87.36 87.21 86.72 86.78 86.72 86.80 86.44 86.16 85.91

710 711 692 734 717 719 714 730 722 737 759 762 765 763 788 805 824

5.01 5.14 4.94 5.03 5.18 4.99 5.07 5.06 4.97 4.89 4.84 4.94 5.06 4.93

.71 .68 .73 .75 .71 .74 .74 .75 .69 .76 .79 .77 .73 .76

86.45 86.27 86.22 85.89 85.82 85.56 85.54 85.36 85.82 85.06 84.85 84.85 84.78 84.41

798 809 818 839 842 863 863 873 852 907 920 921 932 962

14 A y r s h i r e Cows 135 326 304 294 74 3~2 224 354 320 350 321 323 348 351

18 9 9 17 7 9 18 9 9 9 8 18

7 9

595 288 322 584 245 315 645 308 308 315 273 610 245 308

8.4 18.9 10.6 10.5 11.0 8.7 14.2 8.2 6.1 5.5 7.0 7.9 4.8 7.8

3.32 3.46 3.94 3.96 4.00 4.00 4.01 4.18 4.19 4.21 4.37 4.38 4.60 4.96

3.13 3.45 3.39 3.65 3.09 3.43 3.57 3.30 3.54 3.49 3.73 3.64 3.65 3.67

17 B r o w n Swiss Cows 510 479 374 499 426 394 427 435 376 393 395 475 404 439 445 401 48O

10 10 21 9 19 29 30 20 10 20 10 21 20 32 20 10 11

280 280 602 252 532 826 833 560 287 574 280 581 553 875 559 280 301

18.6 15.3 21.5 12.5 14.8 27.8 21.7 24.4 17.2 13.4 16.2 11.6 9.7 16.2

18.7 18.9 14.3

3.38 3.44 3.45 3.50 3.51 3.54 3.62 3.63 3.69 3.90 3.90 3.96 3.99 3.99 4.18 4.28 4.35

3.33 3.44 3.17 3.61 3.41 3.37 3.05 3.38 3.39 3.21 3.32 3.44 3.46 3.48 3.51 3.55 3.96

14 G u e r n s e y Cows 262 284 335 303 282 300 297 271 272 331 267 301 315 270

9 9 9 18 10 18 9 9 10 9 18 19 9 27

309 308 301 631 330 617 309 315 315 301 644 641 308 926

15.9 16.5 18.4 8.2 10.2 8.1 18.1 7.7 9.7 9.4 7.6 7.6 10.1 8.3

4.43 4.49 4.59 4.69 4.72 4.82 4.82 4.86 4.89 5.23 5.27 5.31 5.54 5.74

3.40 3.42 3.52 3.64 3.57 3.89 3.83 3.97 3.62 4.06 4.25 4.13 3.89 4.16

264

w.L.

OAINES AND O. R. OVERMAN

TABLE

l:ierd No.

l - - ( Continued)

Samples

Total period

Milk yield

l~at (f)

Protein (p)

Laetose (1)

No.

"ds.

kg.

%

%

%

Ash

Water

(a)

(w)

Energy p e r kg. milk

%

eal.

%

15 H o l s t e i n Cows 302 273 254 288 325 263 251 324 200 298 295 322 250 257 296

9 8 9 9 9 9 9 9 9 1~

17 9

315 308 315 315 295 315 315 302 315 630 315 309 280 624 315

22.7 21.4 28.7 22.7 21.4 29.2 20.5 24.7 17.1 19.0 16.7 18.2 15.5 20.9 13.8

2.92 3.02 3.04 3.09 3.16 3.19 3.24 3.31 3.36 3.45 3.46 3.50 3.62 3.65 3.67

3.13 2.99 3.03 3.31 3.46 3.05 2.97 3.25 3.09 3.53 3.38 3.61 3.08 3.29 3.73

5.10 4.96 4,63 5.11 5.15 4.56 4.82 5.12 4.46 4.86 4.89 5.01 4.92 5.00 4.96

.65 .63 .68 .66 .67 .66 .67 .65 .67 .70 .71 .70 .67 .67 .71

88.20 88.40 88.62 87.83 87.56 88.54 88.30 87.67 88.42 87.46 87.56 87.18 87.71 87.39 86.93

644 639 633 671 687 644 654 687 658 708 702 723 700 718 744

4.92 5.19 4.99 5.03 5.32 5.16 4.85 5.09 5.21 5.10 4.89 4.95 5.07

.66 .69 ,68 .69 ,70 .69 .70 .68 .72 .73 .74 .78 .69

86.69 85.97 86.13 85.74 85.56 85.53 85.66 85.44 84.98

84.70 84.20 84.10

785 829 824 847 857 863 866 880 906 916 935' 981 987

.73

87.57 87.22 87.31 86.94 87.04 87.00 86.93 86.68 86.20 85.90 86.25 86.86 86.75 85.96 86.31 86.06 86.36 85.63 85.69 85.78 ~5.57

712 728 730 752 747 751 760 774 800 814 799 766 781 825 810 825 803 855 854 856 864

13 J e r s e y Cows 333 336 305 279 341 299 313 327 334 317 314 349 347

9 9 9 9 9 9 17 1~ 18 9 7 8

296 287 312 315 315 349 588 640 308 617 315 238 273

18.3 8.6 11.9

10.5 9.6 9.5 11.7 9.1 11.5 9.0 14.0 4.8 4.1

4.37 4.59 4.61 4.68 4.79 4.85 5.00 5.12 5.21 5.32 5.45 5.95 6.00

3.36 3,56 3.59 3.86 3.63 3.77 3.79 3.67 3.88 3.95 4.22 4.12 4.14

s.4.9o

21 G u e r n s e y - H o l s t e i n 1~ Cows 683 671 665 655 657 663 666 690 680 689 674 673 651 667 668 653 661 688 670 654 659

9 27 9 9 9 18 9 9 9 9 9 9 9 9 8 7 9 18 19 10 9

315 938 308 308 301 618 315 301 315 294 308 315 303 308 280 245 301 610 631 315 3O8

10.4 15.0 17.5 12.8 8.8 16.4 11.0 6.8 10.6 11.5 11.1 16.5 8.5 13.2 10.2 4.9 14.5 10.9 11.2 14.9 11.1

3.64 3.69 3,81 3.85 3.88 3.93 4.04 4.07 4.13 4.13 4.16 4.19 4.26 4.36 4.37 4.40 4.44 4.75 4.78 4.83 4.93

3.38 3.42 3.31 3.66 3.45 3.44 3.50 3.66 3.90 3.96 3.82 3.20 3.60 4,06 3.97 4.23 3,45 3.80 3.78 4.01 3.62

4.68 4,96 4.87 4.78 4.91 4.91 4.77 4.87 5.06 5.28 5.02 5.05 4.68 4.89 4.56 4.53 5.05 5.07 5.02 4.65 5,21

,71 .70 .77 .72 .72 .76 .72 .71 .73 .75 .70 .71 .73 .79 .78 .70 .75 .73 .73 .67

MILK-FAT, ~¢IILK-PROTEIb~ AND MILK-ENERGY YIELD

TABLE

Herd No.

265

1--(Concluded)

Samples

Total period

Milk yield

:Fat (f)

Prot(p) ein

Laetose (1)

No.

ds.

kg.

%

%

%

Ash

Water

(a)

(w)

Energy p e r kg. milk

%

%

cal.

.69 .66 .74 .70 .72 .75 .72 .69 .70 .70 .75 .76 .73 .70 .73 .71 .69 .71 .70 .77 .70 .71 .70 .70 .74

87.22 87.14 86.90 87.25 87.26 87.00 86.92 86.55 86.54 86.10 86.75 86.11 86.47 86.28 86.47 86.15 86.51 85.85 86.14 86.00 85.84 85.99 85.89 85.94 85.53

728 738 751 736 740 755 759 776 787 807 781 811 796 806 80O 815 797 836 819 828 837 833 842 851 881

87.94 87.68 87.50 87.53 87.20 87.33 87.23 86.46 86.76 86.59 86.14

673 688 700 701 718 730 738 778 768 794 825

25 G u e r n s e y - H o l s t e i n F2 Cows 723 729 739 738 702 713 736 707 732 705 710 714 730 735 711 724 737 715 728 742 725 726 720 716 734

19 9 16 10 10 9 8 9 9 9 7 9 9 10 9 18 9 9 9 9 10 9 9 9 8

617 308 553 322 316 315 245 301 308 302 238 308 301 299 309 631 308 309 301 315 308 301 296 302 280

9.6 13.3 9.5 8.5 11.9 8.3 10.9 14.3 8.4 12.6 8.1 10.6 11.4 7.4 10.2 10.4 10.2 7.1 10.6 10.4 11.1 11.6

lO.5 6.4 11.1

3.66 3.87 3.88 3.89 3.94 3.98 4.04 4.04 4.21 4.22 4.26 4.29 4.38 4.39 4.42 4.43 4.46 4.53 4.55 4.64 4.64 4.69 4.74 4.83 5.23

3.52 3.18 3.33 3.35 3.39 3.53 3.35 3.57 3.63 3.77 3.60 3.83 3.43 3.58 3.55 3.70 3.26 3.93 3.54 3.34 3.64 3.56 3.68 4.13 3.54

4.91 5.15 5.15 4.81 4.69 4.74 4.97 5.15 4.92 5.21 4.64 5.01 4.99 5.05 4.83 5.01 5.08 4.98 5.07 5.25 5.18 5.05 4.99 4.40 4.96

11 G u e r n s e y - H o l s t e i n B a c k - C r o s s Cows 806 807 804 801 805 648 802 644 649 803 650

9 9 9 18 9 7 7 9 9 9 7

315 322 315 657 315 259 231 322 295 319 238

10.3 11.2 12.5 11.7 10.5 13.4 8.9 9.9 12.4 11.1 8.8

3.26 3.34 3.41 3.43 3.50 3.77 3.91 4.00 4.07 4.48 4.53

3.11 3.19 3.22 3.28 3.30 3.50 3.26 3.56 3.39 3.26 3.95

5.05 5.11 5.16. 5.06 5.32 4.71 4.92 5.31 5.03 5.02 4.60

.64 .68 .71 .70 .68 .69 .68 .67 .75 .65 .78

half of the individuals, while for the other half it would be incorrect by more than .085 either plus or minus. I f milk yield were constant at 10,000 pounds the corresponding "probable e r r o r " of estimate of 305-day protein yield would be 8.5 pounds. Considering a 305-day protein yield of 300 or 400 pounds it might seem good enough if the estimate is correct within 8 or 9 pounds for half of the individual cows, and correct within 25 or 30 pounds for any individual in the whole population. However, we do not know that the present lot of 130 cows is representative of dairy cows in general, and use of the equation is not recommended, except as an expedient to obtain a rough idea of protein yield.

266

W . L . GAINES AND O. R. OVERMAN

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Fro. 1. Per cent protein (p) plotted against per cent fat (f) for 305-day samples of 130 cows. The c o r r e l a t i o n is .r = .755, a n d the r e g r e s s i o n e q u a t i o n , r e p r e s e n t e d b y t h e s t r a i g h t line, p = 2.10 + .346f.

I n Table 2 the protein-fat percentage equations are given by breed groups, both for the present 305-day samples and the earlier (mixed stages of lactation) 3-day samples. It may be noted from Table 2 that the 305-day protein TABLE 2 Protein percentage according to breed and f a t percentage P r o t e i n p e r c e n t a g e ( p ) as r e l a t e d t o f a t p e r c e n t a g e ( f ) 3-day s a m p l e s

Breed

Equation A y r s h i r e ................................. B r o w n Swiss ........................ G u e r n s e y (G) ..................... H o l s t e i n ( H ) ..................... J e r s e y ....................................... G - H lV1 's .............................. G - H F~ 's ............................. G-l:[ B a c k Cross ............ G - H Cross B r e d ............

3OS-day s a m p l e s p*

.366f .523f .447f .653f .282f

3.58 3.53 4.02 3.42 3.86

p = 1.623 + .499f

3.80

p p p p p

= = = = =

2':061 + 1.509 + 1.699 + 1.100 + 2.402 +

p* 3.48 3.41 3.81 3.26 3.81 3.68 3.56 3.36

Equation p p p p p p p p

= = = = = = = =

2.257 2.091 .866 1.514 1.590 2.112 2.481 2.018

+ .298f + .350f +.594f + .527f + .438f + .371f + .249f + .355f

* A t m e a n f a t p e r c e n t a g e f o r t h e breed.

percentage is lower than the corresponding 3-day protein percentage. As between the several breed groups the protein equations show considerable divergence, the greatest contrast being between the Guernsey, p = .866 + .594f, and the Guernsey-Holstein F2's, p = 2 . 4 8 1 + .249f. The average of the eight 305-day breed equation constants gives p = 1.87 + .398f as compared with p = 2.10 + .346f for the 130 cows as a single group. In the general equation p = a + bf we are inclined to think it probable that b has a value of about .4, in spite of the fact that it works out at .346 in the particular case of these 130 cows.

267

MILK-FAT, MILK-PROTEIN AND MILK-ENERGY YIELD REI.~TION B E T W E E N FAT AND ENERGY

In Figure 2 milk energy per kilogram of milk is plotted against fat percentage. The correlation between fat percentage and calories for the 305-day samples of the 130 cows is r = .9847. The relation between these two is much i

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FzG. 2. Calories of milk energy per kilogram of milk ( E ) plotted against f a t percentage (f) for 305-day samples of 130 cows. The correlation is r--.9847, and the regression equation, represented by the straight line, E = 304.8 + 114.1f.

closer than that between fat and protein per unit of milk ( r = .755) which is natural since fat itself is a direct and dominating contributor to milk energy, while the connection between fat and protein is indirect. The regression equation is E =304.8 +114.1f or E =114.1 (2.671+f). This compares with the equation previously found (4) from 1999 3-day samples (which did not include the Brown Swiss data) E = 115.33 (2.51 + f). It may be noted that the present 305-day formula agrees very closely with the estimate of milk energy in terms of 4-per cent milk by the formula .4 × milk + 15 × fat or 4-per cent milk proportional to (2§ + f). In terms of calories, however, the present equation gives 761 calories per kilogram of 4-per cent milk in comparison with 751 calories by the older (4) equation. It appears therefore that the formula for estimating 305-day energy yield in terms of 4-per cent milk needs no revision, but to convert a kilogram of 4-per cent milk by the .4M + 15F formula to calories for the 305-day period the factor 761 is indicated, instead of the factor 751 as found from the 3-day samples at mixed stages of lactation.

268

W. L. GAINES AND 0. R. OVERMAIq

RELATION BETWEEN PROTEII~AND ENERGY I n F i g u r e 3 milk energy per kilogram of milk is plotted against protein percentage. The correlation between protein percentage and calories for the 305-day samples of the 130 cows is r = .832. The relation between these two is not as close as that between fat percentage and calories (r = .9847) which ¢

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FIG. 3. Calories of milk energy per kilogram of milk ( E ) plotted against grams protein per kilogram of milk ( P ) for 305-day samples of 130 cows. The correlation is r=.832, and the regression equation represented by the heavy s t r a i g h t line, E = 37.8 + 21.0P. The light straight line represents the equation E = 22.6P, derived from the means of E and P alone.

m a y be a reflection of the fact that the protein itself is a smaller contributor to the milk energy t h a n is the f a t itself.

MILK-FAT, MILK-PROTEIN AND MILK-ENERGY YIELD

269

The expression of milk energy as a function of the protein (P) of the milk is given in F i g u r e 3 b y two equations: E = 37.8 + 21.0P and E = 22.6P. The first is the usual least-squares linear regression equation, y = a + bx; the second eliminates the a constant or is simply y = bx adjusted b y least squares, that is, b = mean of x divided by the mean of y. As m a y be seen f r o m the plot of F i g u r e 3 there is a v e r y little difference in the accuracy with which the two equations represent the observations. I n a gross w a y f a t and energy are more closely related ; but in a finer w a y protein and energy m a y be more closely related, if one is a simple multiple of the other. The simple-multiple relation of protein and energy provokes speculation as to the nature of the relationship. I f we m a y say t h a t milk-protein y i e l d is proportional to the nitrogen metabolism of the m a m m a r y gland in lactation, a n d milk-energy yield, to its energy metabolism, then we m a y advance the thought that the functioning of the gland, as measured by its total energy transformations, is geared to and dependent upon a mechanism of protein growth. This conception fits into the old (discredited) theory that milk formation is accomplished b y a process of cell multiplication and disintegration. I t m a y more reasonably be taken to mean t h a t the energy transformations of the milk secreting cell are dependent upon a mechanism of protein elaboration. ( I n this connection compare the work of Brody, Procter and Ashworth (5) showing that the " b a s a l " energy transformations of various species of animals are proportional to their " b a s a l " nitrogen metabolism.) B y t h e equation, energy metabolism of lactation = nitrogen metabolism of lactation × constant, we reach the conclusion t h a t without nitrogen metabolism of the m a m m a r y gland lactation ceases. The milk of all m a m m a l s contains protein, so there is no p a r t i c u l a r instance of contradiction of this conclusion in nature. On the other hand we do have some p a r t i c u l a r cases (e.g., the mare) in which the milk is nearly f a t free, yet lactation proceeds undisturbed and p r e s u m a b l y in accordance with the same protein-energy relation. Taking this fat-free milk as origin we m a y r e g a r d fat-rich milk as the p r o d u c t of the original fat-free mechanism plus an additional one (or acceleration of an original weak one) in which we have a simple multiple relation between fat, protein and energy. Again, we m a y say that a protein mechanism underlies the energy transformations t h a t result in the f o r m a t i o n of milk fat. The milk-protein yield of d a i r y cows thus takes on a special significance. F u r t h e r m o r e , if we accept the generalization that, as between individual hard-working cows, milk-energy yield tends to be independent of milk composition, then we m a y deduce the generalization t h a t milk-protein yield tends to be independent of milk composition. Commercially f a t yield has been forced on our attention. Biologically it seems t h a t protein yield is more deserving of attention t h a n is f a t yield. A f t e r all, however, we have the f o r t u n a t e circumstance that the total work of lactation can be accurately

270

W . L . G A I N E S A N D O. R. O V E R ~ A N

estimated empirically from the common determinations of milk and fat dictated by commercial necessity. P R O T E I N / C A L O R I E R A T I O I N R E L A T I O N TO F A T P E R C E N T A G E

While the foregoing section has placed emphasis on a constant protein/calorie ratio this constancy is only approximate. On the basis of the 3-day samples the equations relating protein per calorie to fat percentage have been reported (4) and are here repeated in Table 3 together with the TABLE

3

Protein eolorie ratio aevord~ng to breed and fat percentage (f) P' = Milligrams of protein per calorie of milk energy Breed

A y r s h i r e ................................. B r o w n S w i s s ..................... G u e r n s e y ( G ) .................. H o l s t e i n ( H ) .................. J e r s e y .......................................

G-H Fl's G-H G-H G-H

3-day samples

305-day

Equation

PP*

pt.

p t = 56.29 - 2.32f

46.7

P ' = 48.89 - . 8 3 f PP = 46.49 + . 4 6 f P ' = 55.59 - 2 . 3 2 f

44.6 48.1 43.6

45.7 45.9 43.7 47.9 43.1 46.5 44.7 46.3

pr = 49.68-

47.0

..............................

1~ 's .............................. B a c k C r o s s ............ C r o s s B r e d ............

.61f

samples

I

Equation P" pt P' pt PP pt P~ P~

= = = = = ± = =

58.92 54.77 40.47 + 51.44 47.0155.35 57.23 54.62 -

3.22f 2.34f .66f 1.08f .78f 2.09f 2.90f 2.19f

At mean fat percentage for the breed.

corresponding equations for the present 305-day samples. In general as fat percentage increases protein per calorie tends to decrease slightly. In the 3-day samples the Holstein breed seemed to be an exception to the general rule. It is therefore of interest to note that in the 305-day samples the exception disappears, and it seems safe to say that for all breeds there is a slight tendency for the amount of protein per calorie to decrease with increase of fat percentage. BREEDINGTO ALTERTHE PROPORTI01~OF PROTEIN AS above noted one object of the American Dairy Cattle Club is to promote the breeding of cows in which the milk protein constitutes a larger proportion of the total food value of the milk. Taking energy as a measure of the total food value of the milk, the object is to increase the protein/calorie ratio. From what has been said this object appears difficult, and perhaps in conflict with the principles of the life processes involved in milk secretion. Still, in Figure 3 it is seen that at a given value of calories per kilogram of milk there is a considerable range in the amount of protein per kilogram of milk, That is, the protein/calorie ratio varies to a certain extent as between the individual cows represented in the present 305-day records. Of the present records 32 cows have two lactations represented. The correlation between the first and second lactations with respect to the protein/calorie ratio for these 32 cows is r = .28 ± .11. A part of that correlation is associated with the fat percentage and if fat percentage is held

MILK-FAT, ~IILK-PROTEIN AND MILK-ENERGY YIELD

271

constant the partial correlation reduces to .18 ± .11. I f individual differences between dairy cows in the protein/calorie ratio are no more stable than indicated by these low correlations any program of altering the protein/calorie ratio of the milk by selective breeding appears rather hopeless. SUI~INIARY AND CONCLUSIONS

The data examined consist of the 305-day partial lactation yields of 130 cows with respect to milk fat, milk protein and milk energy. The yields were determined by continuous milk weights and complete chemical analysis of 3-day samples at 5-week intervals. Where only the milk yield and fat yield are known milk energy yield may be estimated more accurately (r=.985) than can protein yield (r=.755). The accuracy of estimate of energy yield from milk and protein yield is intermediate (r=.832). These correlations are between actual and estimated yields, at a given milk yield. While the correlation between fat percentage and energy per kilogram of milk is much higher ( r = .985) than that between protein percentage and energy (r = .832) the protein-energy relation is regarded as the more significant biologically. This point of view is based on the fact that energy yield tends to be a simple multiple of protein yield. If there is no elaboration of milk protein there are no lactation energy transformations and there is no milk secretion. On the other hand, elaboration of milk fat may be zero without interrupting milk secretion. The elaboration of milk fat recLuires the elaboration of milk protein additional to that of fat-free milk secretion. In general, the total (and often enormous) energy transformations of milk secretion depend on and are proportioned to the elaboration of milk protein or nitrogen metabolism of the mammary gland in lactation. According to the above interpretation it appears futile to t r y to modify the protein calorie ratio of milk by selective breeding. The protein calorie ratio has a low variability (C.V. = 5) and as between successive lactations of the same cow it shows a low correlation (r = .18). Hence, to increase the proportion of food value (calories) present in the milk as protein, by breeding, would be exceedingly difficult. •I ~ E F E R E N C E S (1) GOODALE,]~. D. Elements of value in milk. H o a r d ' s Dairyman, 82: 316-317. 1937. (2) OVER~AN, O. R., SAN~ANN, F. P., AND WRIGHT, K . E . Studies of the composition of milk. Univ. of Ill. Agr. Exp. Sta. Bul. 325. 1929. (3) OVEa~AN, 0. R., WmGH% K. E , AND GARRETT, O. 1~. Studies of the composition of milk of Brown Swiss cows. Univ. of Ill. Agr. Exp. Sta. BuL ( I n press.) (4) OVEX~AN, O. R., Am) GAINES, W . L . Milk energy formulas for various breeds of cattle. Jour. Agr. Res. 46: 1109-1120. 1933. (5) ERODY,SAMUEL, PROCTER,ROBERTC., AND ASHWORTH, URALS. Growth and development X X X I V , Basal metabolism, Endogenous nitrogen, creatinine and neutral sulphur excretions as functions of body weight. Univ. of Mo. Agr. Exp. Sta. Res. Bul. 220. 1934.