Effects of Stage of Maturity and Frost on Nutritive Value of Corn Silage for Lactating Dairy Cows1

Effects of Stage of Maturity and Frost on Nutritive Value of Corn Silage for Lactating Dairy Cows1

Effects of Stage of Maturity and Frost on Nutritive Value of Corn Silage for Lactating Dairy Cows z N. R. ST-PIERRE, R. B O U C H A R D , G. J. S T - ...

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Effects of Stage of Maturity and Frost on Nutritive Value of Corn Silage for Lactating Dairy Cows z N. R. ST-PIERRE, R. B O U C H A R D , G. J. S T - L A U R E N T , 2 C. V I N E T , and G. L. R O Y Agriculture Canada Station de Recherches C.P. 90 Lennoxville, Quebec, Canada J1M 1Z3

INTRODUCTION

ABSTRACT

Nutritive value of corn silage grown under climatic conditions in which frost does not interfere with the growing season is little affected by stage of maturity of the plant (8, 20). Under such conditions, dry matter digestibility plateaus when the whole plant reaches 28% dry matter (7, 10, 28, 29, 33). In the range of 28 to 34%, nutritive value seems not to be affected by stage of maturity (2, 21, 22). At a higher percentage of dry matter, the trend is for a small decrease of nutritive value of corn silage ( 5 , 8 , 14, 16, 22, 25). Corn grown under more nordic climatic conditions often is affected by frost before reaching the dent stage of maturity. Calder et al. (6) observed 8% decrease of dry matter digestibility of corn silage harvested after first frost as measured on beef steers. As described by White et al. (32), frost decreased percentage of nitrogen, phosphorus, and potassium in the whole plant. These losses are explained only partly by the loss of leaves. These authors also observed a 2 to 3% decrease of in vitro digestibility of dry matter of silage harvested after first frost. If these results hold for dairy cows, then the National Research Council (NRC) tables (24) should not be utilized for calculation of a dairy ration, and present recommendation for the optimal stage at harvesting needs to be reassessed for the northern limits of the growing area. This study was to determine effects of frost before harvest on nutritive value of corn silage for lactating dairy cows.

Nutritive value of corn silage harvested before and after frost was evaluated through 12 lactating Holstein cows during five 14-day periods in a feeding and digestibility trial. Silages from corn harvested at milk stage, dough stage, and after one, two, and six frosts were evaluated with control corn silage at dough stage. Concentrate feed made up 30% of the ration dry matter. Percentages of dry matter, neutral detergent fiber, acid detergent fiber, acid detergent lignin, nitrogen, phosphorus, potassium, calcium, and magnesium varied among silages. There was no significant difference of dry matter intake or of nutrient digestibility of the control silage among the five periods, indicating no time change. Production of 4% fat-corrected milk decreased steadily 9% per mo. For milk stage, dough stage, and after one, two, and six frosts, respective intakes of dry matter were 15.1, 16.3, 14.9, 16.6, 14.7 kg/day; 4% fatcorrected milk yields were 17.7, 18.1, 17.8, 17.6, 17.5 kg/day; and milk fat was 3.5, 3.6, 3.4, 3.2, and 3.3%. The partitioning of energy, nitrogen, calcium, phosphorus, potassium, and magnesium favored the feeding of corn silage harvested after two frosts. Apparent digestibilities of dry matter, energy, and nitrogen were reduced by frost for corn harvested as silage after frost.

MATERIALS AND METHODS

Received June 11, 1982.

1 Contribution No. 135. 2D6partement de Zootechnie, Universit6 Laval, 8te-Foy, Qu6bec, GIX 7P4, Canada. 1983 J Dairy Sci 66:1466-1473

Corn cultivar Pride 1124 (2500 CTU), seeded at 85,000 plants per hectare in a 2350 corn thermal unit (CTU) area, was harvested as silage on five dates corresponding to milk stage,

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MATURITY AND FROST OF CORN SILAGE dough stage, and after first, second, and sixth frost (Table 1). Silages were stored in a 4.3 x 10 m vertical silo in five consecutive layers of 20 tons. Silages were separated by approximately 12 cm of wood chips between each layer. Eight lactating Holstein cows, on an average on day 92 of their fourth lactation and producing 26 kg of milk, were fed the five corn silages during consecutive 14-day periods; the ration was changed at the beginning of each period, starting with silage harvested after six frosts through to silage harvested at milk stage. Four other cows, comparable for stage and number of lactations and producing 24 kg of milk, were a control group for the length of the experiment. They were fed a standard corn silage harvested at dough stage. The purpose of the control group was to assess any effect of period (time change) on cow performance. Silages were consumed ad libitum with concentrate representing 30% of the total dry matter. The concentrate contained rolled barley 30%, rolled oats 10%, cracked corn 21.5%, soybean meal 27%, molasses 5%, dicalcium phosphate 3.5%, calcium carbonate 196, cobalt iodized salt 1%, vitamin-mineral premix 1% containing per kilogram 220,000 IU of vitamin A, 55,000 IU of vitamin D3, 5,500 IU of vitamin E, 8.5% salt, 1.0% Fe, .29% Zn, .04% Mg, .05% Ca, and .035% Co. Amounts of concentrate were adjusted every other day for total intake of dry matter of the previous 2 days. Cows housed in a tie-stanchion barn had free access to water and were fed twice daily. Feed weights were recorded at each feeding. Milk was weighed twice daily and feed refusals once. A digestibility trial was conducted during the last 5 days of each experimental period on

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all 12 cows by the procedure described by Vinet et al. (30). Cows were weighed in the morning, twice before and twice after each experimental period. Nitrogen and phosphorus were determined' in corn silage, concentrate, and feed refusals by the procedure of Thomas et al. (27), in feces and urine by that of Cox and Harmon (9) and in the milk by that of Millie et al. (23). All samples were analyzed for potassium, calcium, and magnesium by atomic absorption. Milk fat was determined on a MilkO-Scan apparatus by the infrared technique. Acid detergent fiber (ADF), neutral detergent fiber (NDF), and acid detergent lignin (ADL) were analyzed according to Goering and Van Soest (15). The data were analyzed statistically by the following model: Y i j = P + a i + ~f + eli

where a i = effect of the ith silage /3] = effect of t h e j th cow el'/ = random residual for the ith silage and jth cow. A modification to the above model was used in the analysis of milk production: Y.. + (Yii --~i.) =/~ + c~t +/3j +eij where -Pi. = average milk production of the control group for the ith period Y.. = average milk production of the experimental group throughout the experiment.

TABLE 1. Stage of maturity, date of harvest, and date of frosts for corn harvested as silage.

Stage

Date of harvest

Date of frosts

Milk Dough After the first frost After the second frost After the sixth frost Control--Dough

September 9 September 22 October 7 October 11 October 26 October 5

... ... October 5 October 5, 8 October 5, 8, 14, 16, 24, 25 ...

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A Duncan multiple range test (12) was used whenever the F-test was significant (18) to denote treatment differences. RESULTS A N D DISCUSSION Silage Composition

Chemical compositions of silage and concentrate are in Table 2. Dry matter content increased from 19.8% for the milk stage to 35.6% for the sixth frost silage. The relationship between dry matter content of the silage (DMS) and date of harvesting, expressed as the number of days between September 1 and the day of harvest (d), is described by the following linear equation: DMS = .332 d a y s - 16.52, with r=.99. Thus, frost does not influence rate of dehydration of the corn plant. This confirms results by Aboul-Ela (1) and Hicks et al. (19). Decrease for nitrogen and calcium concentrations in silages as a function of maturity was linear. Content of ADF decreased from 36.4% in the milk stage silage to 28.5% in the first frost silage, and increased to 30.4% in the sixth frost silage. Trend was similar for NDF. Under conditions of a long frost-free season, Weaver et al. (31) observed a decrease of ADF and NDF concentrations, until the whole plant reached dry matter 36%, and subsequently fiber concentration increased with increasing dry matter content. Frost seems to influence fiber concentration by hastening the minimal fiber reached in development of the plant. This may be due to loss of leaves, which contain less fiber than

the stalk. Magnesium and phosphorus concentration in silages remained constant. Calcium content was affected by frost. Decrease was substantial for silage harvested after first frost, when calcium decreased from .54 to .34% dry matter. Potassium content varied greatly between silages. The high solubility of this element in the corn plant makes it difficult to distinguish effects of field conditions prior to harvest from effects of downward migration between layers in the silo. Performance Data and Digestibility Studies

Control Diet. Data from control cows indicate that there were no effects of period on measures studied except for milk production which decreased 9% per mo for the length of the experiment (Table 3). This is normal decrease of milk production. Uncontrolled factors associated with period of experimentation had few if any effects on experimental cows. Means among treatments can be compared directly except for milk production to which a factor was applied to account for the 9% decrease. Cows weighing an average of 552 kg consumed the control silage at 2.0% of their body weight. This is less than the 2.3% expected for this production (26). Apparent dry matter, gross energy, and nitrogen digestibilities averaged 59.1, 61.0, 49.1% and are indicative of a corn silage of poor quality (7). The NDF, ADF, and lignin apparent digestibilities averaged 50.8, 44.6, and 20.6% over all five periods. Average apparent digestibilities of minerals were 26.8%

TABLE 2. Chemical composition of corn silages and concentrate feed.

Item Dry matter Neutral detergent fiber 59.4 Acid detergent fiber Acid detergent lignin Nitrogen Phosphorus Potassium Calcium Magnesium

Experimental silages Dough First Second stage frost frost

Milk stage 19.8

24.0 57.6

36.4 3.9 1.66 .20 1.35 .60 .14

33.3 3.9 1.50 .17 1.04 .54 .10

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27.6 52.2 28.5 3.9 1.43 .17 .77 .34 .11

30.8 54.4 28.7 4.0 1.44 .19 .99 .32 .11

Sixth frost

Control silage Dough stage

35.6 59.1 30.4 4.1 1.37 .19 .85 .25 .10

27.4 57.3 29.3 3.8 1.45 .18 1.16 .36 .14

Concentrate 87.5 23.2 7.6 1.9 3.33 1.05 1~27 .86 ~26

MATURITY AND FROST OF CORN SILAGE

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TABLE 3. Effect of experimental periods on body weight (BW), dry matter intake (DM1), milk production, milk composition, and apparent digestibility of nutrients of control diet. Period Item BVq, kg DMI, kg/day DMI, % of BW 4% Fat-corrected milk, kg/day Milk fat, % Milk solids-not-fat, % Apparent digestibility, % Dry matter Gross energy Nitrogen Neutral detergent fiber Acid detergent fiber Acid detergent lignin Phosphorus Calcium Potassium Magnesium

1

2

3

545 16.1 3.0 19.3 a 3.15 8.86

544 16.2 3.0 18.lab 3.12 8.77

545 16.1 3.0 17.8 b 3.18 8.74

59.8 61.8 49.8 53.6 46.6 28.3 26.8 29.1 51.5 23.2

59.0 60.6 50.8 52.0 46.9 21.0 30.6 27.4 47.7 19.8

58.6 60.2 48.4 50.3 41.9 20.1 26.3 26.1 49.9 17.7

4

5

Mean

SE

560 16.1 2.9 17.2bc 3.20 8.82

566 16.0 2.8 16.3 c 3.22 8.86

552 16.1 2.9 17.7 3.17 8.81

1.8 .03 .03 .81 .05 .09

58.7 60.9 49.9 49.1 45.9 17.1 23.8 28.4 46.2 18.6

59.4 61.5 46.7 49.1 41.8 16.7 26.7 27.8 47.2 17.9

59.1 61.0 49.1 50.8 44.6 20.6 26.8 27.8 48.5 19.4

.4 .4 .8 .8 1.1 2.3 1.5 1.2 1.2 1.2

a'b'CAny two means in a row followed by different letters differ (P<.05).

for phosphorus, 27.8% for calcium, and 19.4% for magnesium. These percents compare with those in the review by Gueguen (17), w h o rep o r t e d digestibility of P, Ca, and Mg 43, 30, and 25%. Digestibility of potassium was 48.5%, which is m u c h lower than the 90% r e p o r t e d in the review. Experimental Diets. The decrease of b o d y weight (P<.05) for silage harvested at milk stage (Table 4) was a consequence of intake problems during the adaptation period. Cows, consuming silage harvested at milk stage, dough stage, and after second frost gained weight, indicating that these rations were m o r e than sufficient for milk p r o d u c t i o n . When silages harvested after second and sixth frost were fed, cows lost weight, which led us to believe that under these conditions the rations were n o t sufficient to support milk p r o d u c t i o n . Dry m a t t e r intake (DMI) was affected by t r e a t m e n t s (P~<.05). The lowest feed intake was 14.7 kg/day or 2.8% of b o d y weight and was for silage harvested after sixth frost, whereas the highest feed intake was 16.6 kg/day or 3.1% of b o d y weight for silage harvested after second frost. These data present a different trend f r o m that for corn silage g r o w n u n d e r frost-free con-

ditions. In a review by St-Pierre (26), for corn not subject to frost, DMI increased linearly for silage within the 20 to 32% dry m a t t e r range, levels off b e t w e e n 32 and 36% dry matter, and slightly decreases above 36% dry matter. Stage of m a t u r i t y affects silage intake such t h a t intake of silage subjected to frost differs m a r k e d l y f r o m that of frost-free silage. If silage harvested at dough stage is taken as standard, its relative intake to a h y p o t h e t i c a l dent-stage silage of 32 to 34% dry m a t t e r is 85%. Similarly, relative intakes of silage harvested at the milk stage and after one, two, and six frosts are 76, 7 5 , 8 5 , and 74%. The 76% for silage harvested at the milk stage agrees with (4, 13, 14). The effect o f the first frost o n intake is m o r e i m p o r t a n t than simply the difference o f intake b e t w e e n silages at the dough stage and the one harvested after one frost. The first-frost silage ought to have been c o n s u m e d at relative intake of 95% or 2.5% of b o d y weight. Thus, intake of silage harvested after one frost was 10% lower than a dough-stage silage harvested u n d e r frost-free conditions; f u r t h e r m o r e , it was 20% lower than a h y p o t h e t i c a l silage of equal m a t u r i t y not subjected to frost. This decline was 14 and 26% for silages harvested after two Journal of Dairy Science Vol. 66, No. 7, 1983

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TABLE 4. Effect of stage of maturity of corn silage and number of frosts on body weight (BW), dry matter intake (DM1), milk production, and milk composition. Stage of maturity of silage

No. of frosts at harvesting

Item

Milk

Dough

First

Second

Sixth

SE

BW, kg BW gain, kg/day DMI, kg/day DM1, % of BW 4% FCM, kg/day Milk fat, % Milk solids-not-fat, %

524 a + 1.0 a 15. Iab 2.9bc 17.7 3.5ab 8.8bc

539 b +1.3 a 16.3 b 3.0ab 18.1 3.6 a 8.6 c

538 b - 1.0 b 14.9 a 2.8 c 17.8 3.4bc 8.9ab

532 b +.7 a 16.6 b 3.1 e 17.6 3.2 c 9.0 a

532 b -.8 b 14.7 a 2.8 c 17.5 3.3 c 8.7 c

1.0 .5 .1 .03 .6 .03 .04

a'b'c'dAny two means in a row followed by different letters differ (P<.05).

and six frosts. These results agree with those Calder (6) observed on steers. Silage intake also is affected greatly by products of f e r m e n t a t i o n . As corn matures and dry m a t t e r c o n t e n t reaches an o p t i m u m 30 to 35%, there is less effluent p r o d u c e d within the silo, resulting in better f e r m e n t a t i o n . Consequently, intake is improved (3). The higher fiber and lignin contents of silage harvested after six frosts can explain the decreased DMI of that silage (Table 2). Milk p r o d u c t i o n was not affected by stage of m a t u r i t y of corn silage (Table 4). That may be due to the short experimental period that prevented noticeable differences because cows were able to maintain milk p r o d u c t i o n by mobilizing b o d y reserves. However, t r e a t m e n t s had a significant effect on milk composition. Milk fat percentage reached its highest at 3.6% for silage harvested at dough stage. This was higher (P<~.05) than 3.2% and 3.3% for silages harvested after two and six frosts. Partitioning of energy, nitrogen, calcium, phosphorus, potassium, and magnesium is in Table 5. Percent energy digested was highest for silage harvested after dough stage and lowest for silage harvested after six frosts. The digested nitrogen was higher (P~<.05) for silages harvested before frost than for those harvested after frost. This is probably due to the higher nitrogen c o n c e n t r a t i o n in silages harvested at the milk and dough stages (Table 4). Nitrogen r e t e n t i o n was affected by stages of corn silage. It decreased f r o m a positive balance of 15 g/day with silage harvested at the milk stage to a negaJournal of Dairy Science Vol. 66, No. 7, 1983

rive balance of 13 g/day for silage harvested after sixth frost. Calcium r e t e n t i o n (CAR) showed the same general trend with a negative 3.2 g/day for silage harvested after sixth frost. The relationship b e t w e e n CaR and calcium intake (CaI) is best described by the following linear e q u a t i o n : CaR (g/day) = .343 Cal (g/day) 23.3, r=.99. F r o m the data, CaR can be evaluated to be zero w h e n a 550 kg cow, producing 18 kg/day of 4% FCM, is consuming 67.9 g/day of dietary calcium. This 67.9 g/day is in c o n c o r d a n c e with the 68.6 g/day recomm e n d e d by the N R C (24). T r e a t m e n t effect was significant (P~<.05) for phosphorus intake (PI) although variation a m o n g t r e a t m e n t s was small because 70% of the total ration phosphorus was provided by the concentrate. The difference for PI was more affected by a m o u n t of concentrate ingested than by c o m p o s i t i o n of silage. Phosphorus retained (PR) was positive for all t r e a t m e n t s (P~<.05). The linear relationship between PR and PI is described by the equation: PR (g/day) = .606 PI (g/day). - 33.6, r=.67. F r o m that equation, a 550 kg cow producing 18 kg/day of 4% FCM would need 55.4 g/day of dietary phosphorus. This is slightly higher than the 48 g r e c o m m e n d e d by the NRC (24). Potassium intake (KI) was affected (P<~.05) by stage of m a t u r i t y of corn silage. The highest intake was for silage f r o m corn at milk stage, and the lowest was for silage harvested after first frost. The relationship b e t w e e n potassium in urine (KU) and potassium digested (KD) is described by the following linear e q u a t i o n : KU (g/day) = .827 KD (g/day) - 26.8, r=.99. -

MATURITY AND FROST OF CORN SILAGE This confirms the generaI theory that potassium is r e g u l a t e d b y k i d n e y , a n d its c o n c e n t r a t i o n in u r i n e is a g o o d i n d i c a t o r o f k i d n e y s t a t u s . Potassium retained by the tissues (KR) was p o s i t i v e f o r all t r e a t m e n t s a n d m o d e r a t e l y w a s explained by potassium intake (KI) according t o t h e f o l l o w i n g linear e q u a t i o n : K R ( g / d a y ) = . 0 1 7 0 K I ( g / d a y ) - 18.9, r = . 7 4 . T h u s , p o t a s s i u m r e q u i r e m e n t s o f a 5 5 0 kg c o w , p r o d u c i n g

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18 kg/day" o f 4°6 FCM, w o u l d b e 112 g / d a y o r .75% o f t h e t o t a l r a t i o n , w h i c h is e q u i v a l e n t t o t h e .8% p r o p o s e d b y D e n n i s et al. (11). Magnesium was retained positively by tissues ( M g R ) f o r all t r e a t m e n t s . F r o m t h e d a t a , a 5 5 0 kg c o w , p r o d u c i n g 18 k g / d a y o f FCM, w o u l d n e e d a daily a l l o w a n c e o f 22 g o f m a g n e s i u m o r .15% o f d i e t a r y i n t a k e . T h a t a l t o w a n c e is b e l o w t h e .2% p r o p o s e d b y t h e N R C ( 2 4 ) b u t

TABLE 5. Effect of stage of maturity, of corn silage and number of frosts on partitioning of the nutrients. No. of frosts at harvesting

Stage of maturity Item

Milk

Dough

First

Second

Sixth

SE

Gross energy, MJ/day Intake Digested Z Urine Milk

304.3 c 195.9 a 10.5 55.7 ab

339.5 a 230.2 h 9.2 59.9 a

295.1 c 199.3 a 11.3 57.3 ab

319.0 b 204.3 a 10.9 59.4 a

278.4 d 172.9 c 10.5 54.4 b

2.1 1.7 .3 .4

Nitrogen, g/day Intake Digested Urine Milk Tissues

325.2 a 194.6 a 83.0 a 96.6 b 15.0 a

325.7 a 185.2 a 80.0ab 94.4 b 10.8ab

285.2 b 154.6 b 56.8 d 96.2 b 1.6bc

298.6 b 161.8 b 64.6 cd 101.4 a -4.2 c

287.6 b 152.6 b 71.8 bc 93.8 b -13.0 d

1.8 2.1 1.5 .7 2.8

Calcium, g/day Intake Digested Urine Milk Tissues

100.6 a 37.2 ab 5.8 a 20.9 a 10.6 a

102.0 a 39.8 a 7.2abc 20.3a 12.4 a

77.2 b 31.2 b 8.6 c 20.4 a 2.2 b

72.6 c 33.6 ab 6.4 ab 24.4 b 2.6 b

58.8 d 23.8 c 7.7 bc 19.2 a -3.2 b

,5 1.0 .2 .3 1.0

62.4 c 20.8 b .18 b 15.5 b 5.0

68.8 a 27,2 a .28 ab 17.4 a 9.6

66.4 ab 18.2 b .34 a 14.1 b 3.8

.4 .9 .02 .3 1.1

Phosphorus, g/day Intake Digested Urine Milk Tissues

67.2ab 22.8 ab .20 ab 15.4 b 7.4

Potassium, g/day Intake Digested Urine Milk Tissues

196.0 a 111.2 a 71.0 a 30.6ab 9.6ab

Magnesium, g/day Intake Digested Urine Milk Tissues 1 Intake

of nutrient

26.9 a 6.5 b 3.9bc 1.6 a 1.0 -

64.4bc 21.2 ab ,24 ab 15.2 b 5.8

174.8 b 112.6 a 65.8 a 29.4ab 17.4 a

22.2 c 4.5 c 3.1 c 1.5 a .0

121.4 c 44.0 c 8.8 c 32.6 ab 2.6 c

174.4 b 93.2 b 43.4 b 33.8 a 16.0 a

136.6 c 36.4 c

24.5 b 7.7 b 4.7 ab 1.6 a 1.3

24.1 b 9.1 a 5.6 a 1.7 a 1.8

21.2 c 7,3 b 5.3 a .8 b .8

5.6 c

28.4 b 2.4 c

.9 1.5 1.2 .6 1.9

.2 .2 .2 .04 .3

nutrient in feces.

a'b'c'dAny two means in a row followed by different letters differ (P~<.05).

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TABLE 6, Effect of stage of maturity of corn silage and number of frosts on apparent nutrient digestibility. Stage of maturity

No. of frosts at harvesting

Item

Milk

Dough

First

Second

Sixth

SE

Apparent digestibility, % Dry matter Gross energy Nitrogen Neutral detergent fiber Acid detergent fiber Acid detergent lignin Phosphorus Calcium Potassium Magnesium

62.6 bc 64.5 bc 59.8 a 58.4 57.1 a 25.2 c 34.0ab 37.0 a 56.7 b 24.0 c

66.3 a 68.7 a 57.0 b 56.9 50.3 b 29.1 bc 32.9ab 39.0 ab 64.4 a 20.5 c

65.1 ab 67.2 ab 54.2 c 54.7 51.1 b 31.7 ab 33.3 ab 40.4 ab 36.2 c 31.3 b

63.4 b 64.0 bc 54.2 c 57.4 50.7 b 36.2 a 39.5 a 46.2 b 52.8 b 37.6 a

60.5 c 62.1 c 53.1 c 57.7 49.8 b 29.9 bc 27.3 b 40.4 ab 26.6 d 32.6 ab

.4 .4 .6 .6 .6 .9 1.2 1.1 .9 .8

a'b'c'dAny two means in a row followed by different letters differ (P<.05).

agrees with the .15 to .16% proposed by Gueguen (17). A p p a r e n t digestibility of dry m a t t e r (DMD) was affected significantly by treatment~ (Table 6). The DMD increased f r o m 62.6% for silage at milk stage to 66.3% for dough stage. When corn was harvested after first frost, dry m a t t e r digestibility of the resulting silage declined to 65.1%, and any subsequent frosts accentuated the decline to the p o i n t where the DMD was only 60.5% after sixth frost. First frost is thus responsible for an apparent decline of 1.2%. Results for corn grown under frost-free conditions showed that DMD of corn harvested in the range of 28 to 34% dry m a t t e r was a b o u t 68% (26). Thus, the real effect of first frost is a decline of nearly 3% for DMD. A f t e r sixth frost, the real decline of DMD of silage is estim a t e d at 8%. Trend was similar for apparent digestibility of gross energy. Therefore, corn harvested after frost contains less digestible energy than corn grown u n d e r frost-free conditions. A p p a r e n t digestibility of nitrogen (NAD) was affected (P<~.05) by t r e a t m e n t s (Table 6). Harvesting after first frost resulted in reduced N A D as c o m p a r e d with corn harvested at dough stage. S u b s e q u e n t frost did not accentuate that decline. The relationship b e t w e e n N A D and nitrogen intake (NI) is described by the linear e q u a t i o n : N A D (%) = .123 NI (g/day) + 18.0, r=.90. The apparent decrease of N A D with corn harvested after frost was possibly the conseJournal of Dairy Science Vol. 66, No. 7, 1983

quence of reduced nitrogen intake. The N D F and A D F digestibilities were n o t affected significantly by frost whereas digestibility of A D F in the silage at milk stage was 6.7% higher than in the others. This is possibly due to the greater nitrogen c o n t e n t in that silage. The A D L digestibility was affected by t r e a t m e n t s (P<<..05), increasing f r o m the silage harvested at milk stage through silage harvested after t w o frosts, then decreasing for silage harvested after six frosts. As for control silage, apparent digestibilities of minerals are p o o r indicators of utilization of minerals. Endogenous fecal losses, which are o f t e n i m p o r t a n t and variable (17), are n o t a c c o u n t e d for always. Data of this e x p e r i m e n t indicate that frost affects c o n c e n t r a t i o n of m a n y nutrients of the corn plant such as minerals and fiber. It also affects utilization of nutrients by lactating dairy cows, reducing apparent digestibilities of dry matter, gross energy, and nitrogen. It seems also that frost affects intake of dry matter, b u t o t h e r e x p e r i m e n t s are needed to q u a n t i f y properly the r e d u c t i o n of dry m a t t e r intake attribu t e d to frost before harvest of the corn as silage. ACKNOWL EDGM ENTS

The project was funded Conseil de R e c h e r c h e et des du Quebec (CRSAQ) and Quebec. The experimental

partially by the Services Agricoles Universit~ Laval, w o r k was at the

MATURITY AND FROST OF CORN SILAGE A g r i c u l t u r e C a n a d a R e s e a r c h S t a t i o n in L e n n o x v i l l e , Q u e b e c . T h e a u t h o r s are g r a t e f u l to A . Belleau, S. B o u l e t , a n d G. G i l b e r t f o r t h e i r technical assistance. REFERENCES

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Journal of Dairy Science Vol. 66, No. 7, 1983