The intake and digestion of maize silage-based diets by dairy cows and sheep

Animal Feed Science and Technology, 20 (1988) 299-312

299

Elsevier Science Publishers B.V., AmsterdAm m Printed in The Netherlands

The Intake and Digestion of Maize Silage-based Diets by Dairy Cows and Shvvp J.B. MORAN, C. LEMERLE and T.E. TRIGG*

Kyabram Research Institute, Kyabram, Vic. 3620 (Australia) (Received 10 August 1987; accepted for publication 30 December 1987)

ABSTRACT Moran, J.B.,Lemerle, C. and Trigg, T.E., 1988. The intake and digestionof maize silage-based dietsby dairy cows and sheep. Anita. Feed Sci. Technol.,20: 299-312. Six lactatingcows, 6 dry cows and 6 wether sheep were fed ad libitum on diets of maize silage, maize silageplus lucerne, or maize silageplus lucerne plus wheat. Faeces and urine collections allowed for the determination of digestibilityof dry matter, organic matter and nitrogen, and balances of nitrogen and water. Voluntary feed intakes were highest and digestibilityvalues were lowest in lactatingcows. The addition of lucerne reduced organic matter digestibilityin dry cows, but not in lactatingcows or sheep. The addition of wheat decreased intake in dry cows and sheep, but not in lactatingcows. Production of milk,protein,solids-not-fatand totalsolidsincreasedwith dietaryquality,but there was a depression in milk fat content as a resultof wheat supplementation. The ranking of the 3 dietson the basis of feed intake differedwith each classof livestock,but lactatingcows and sheep gave the same ranking on the basis of organic matter digestibility.

INTRODUCTION

During the last 2 decades, maize silage has become an integral part of dairy production systems in Europe and North America. The Australian dairy industry relies essentially on cows grazing perennial pastures, but there is increasing interest in maize silage both as a supplement to pasture or as the basis of complete diets, particularly on irrigated farms (Trigg and Moran, 1984). In assessing the nutritive value of maize silage for lactating cows, there are various indirect methods available, but at present there is disagreement about their relative accuracy. Wainman et al. (1978) found that the metabolizable energy (ME) content of maize silage could not be accurately predicted from chemical analyses or in vitro digestibility. On the other hand, Deinum et al. (1984) recommended a fixed value of 0.73 dry matter (DM) digestibility in *Present address:Peptide Technology Ltd., P.O. Box 444, Dee Why, N.S.W. 2099 Australia.

0377-8401/88/$03.50

© 1988 Elsevier Science Publishers B.V.

300 vitro for forage maize crops throughout The Netherlands with corrections being made for variations in weather, hybrid selection and cropping practice. Both Van Es (1978) and Lindgren (1981) considered that digestibility data from sheep could be used for predicting the ME content of mixed diets for lactating cows, including those incorporating maize silage; other workers (Tyrrell and Moe, 1975; Thomas and Campling, 1979) have expressed concern over the extrapolation of sheep digestibility data to lactating cows, particularly when compared at different levels of intake relative to maintenance requirements. Comparisons between sheep and dry cows have generally shown closer agreement, probably because of similarities in appetite per unit of live weight (Thomas and Campling, 1979). During an evaluation of forage maize grown under irrigation at Kyabram in northern Victoria and ensiled for feeding to dairy cows, 3 classes of livestock were compared in their ranking of 3 diets (silage, silage plus protein and silage plus protein plus energy) on the bases of voluntary feed intake and digestibility. The objective of this study was to assess whether sheep or non-lactating cows could provide a model for lactating cows in these terms. MATERIALS AND M E T H O D S

Animals and management The study involved 6 lactating crossbred Friesian cows (38 days post-parturn, 395 kg liveweight, 4-5 previous lactations), 6 non-lactating or dry crossbred Friesian cows (5 months pregnant, 490 kg liveweight, 4-5 previous lactations) and 6 Merino wethers {39.3 kg liveweight). Each of 2 diets was fed for a 2-week preliminary and a 1-week collection period. For a third diet, an extra week's preliminary feeding was allowed to accustom animals to the included wheat. Animals were fed as groups for the first week, then individually for the rest of the preliminary period prior to entering metabolism cages for the collection period. Mean within-trial maximum and minimum dry bulb temperatures averaged 22 and 8 ° C, respectively. Diets The 3 diets were based on maize silage grown at the Kyabram Research Institute 6 months previously and harvested at the hard dent stage using a precision chop machine. In the first diet (Diet M), the maize silage was supplemented with urea and minerals at 13 and 10 g kg -1, respectively, on a DM basis; the mineral pre-mix provided additional calcium, phosphorus, potassium and magnesium as well as minor trace elements. The second diet (Diet ML) consisted of a mixture of maize silage and chaffed lucerne (65:33 on a DM basis) with 13 g urea and 10 g kg -1 minerals, while the third diet (Diet

301

MLW) consisted of maize silage, chaffed lucerne and crushed wheat (32:15:50 on a DM basis) with 14 g urea and 10 g kg-1 minerals. All diets were offered ad libitum at 1.15 of the previous day's intakes. Diets were blended in a mixer waggon and offered at 08.00 and 16.00 h with refusals collected prior to the morning feed. The DM contents of feed offered and refused were measured daily using a forced draught oven at 105 °C and drying to constant weight. Measurements

The animals were confined to metabolism cages for 7 days. Each animal was weighed prior to the morning feed at the beginning and end of the collection period to provide a mean liveweight. Urine and faeces were separated in cows using faecal and urine chutes developed from a design described by Hughes (1963). Proportionate samples of urine, faeces, feed offered and feed refused were collected daily and bulked within animals over 7 days. The volume of urine was calculated from its weight and specific gravity, correcting for added sulphuric acid preservative. Daily water intakes were measured. Rumen samples were collected at 07.00 h on the last day of each collection period from each animal using a stomach tube. Rumen fluid pH was measured immediately; the sample was then acidified, centrifuged and analysed for ammonia concentration (Conway, 1957). Lactating cows were milked twice daily, milk being proportionately sampled and bulked over 7 days and analysed for milk fat and protein (Milko Tester MK III and Pro-milk; Foss Electric, Denmark, respectively ) contents and freeze dried to determine total solids. Silage offered and feed residues were sampled, frozen, mixed with dry ice and milled through a 1-mm screen. Dry ice was continually added to the grinding mill to ensure the sample remained below 0 ° C to reduce losses of volatile compounds prior to macro-Kjeldahl nitrogen determinations. Nitrogen (N) TABLE 1 Chemical composition of feed constituents and mixed Diets M (maize silage), ML (Maize silage/lucerne) and MLW (maize silage/lucerne/wheat)

Maize silage Lucerne chaff Crushed wheat Diet M Diet ML Diet MLW

Dry matter (gkg -I)

Organic matter (gkg -~ D M )

Nitrogen (g kg -I D M )

Neutral detergent fibre (gkg-~ D M )

Acid detergent fibre (gkg-~ D M )

362 874 905 370 551 727

891 906 983 871 881 914

13.3 27.2 19.3 17.8 23.6 25.1

498 522 120 487 496 298

350 375 41 342 351 189

302 analyses were also conducted on air-dry ground lucerne and wheat, and frozen faeces and urine samples. Additional feed and faeces samples were oven dried at 60 ° C, ground and ashed for organic matter (OM) determination. Neutral detergent fibre (NDF) was determined with the inclusion of amylase in the detergent mixture for maize silage and wheat (Robertson and van Soest, 1977) and acid detergent fibre (ADF) by the method of Goering and van Soest (1970). The chemical composition of feed ingredients and mixed diets offered are presented in Table 1.

Statistical analyses The data from the 18 animals were analysed using a split plot analysis of variance, main plots being class of animal and sub-plots being diet and interaction between class and diet. Milk production, feed intake and N metabolism data for lactating cows were also tested for dietary effects using analyses of variance. RESULTS Voluntary feed intake and digestibility data are presented in Table 2. Feed intakes were compared on the basis of liveweight to the power 0.90 as suggested by Graham (1972) for studies involving sheep and cattle. The class-by-diet interactions for many of the data in Tables 2, 3 and 4 were statistically significant, so these tables present mean values and statistical comparisons for the 3 livestock classes within each diet and the statistical comparisons for the diets within livestock class. The intakes of both DM and digestible OM were, in decreasing order, lactating cows > sheep > dry cows. Within each livestock class, DM intakes were lower for Diet MLW than for Diet ML. Digestibility of DM and OM was lowest in lactating cows on all 3 diets. Digestibility values for DM and OM were highest in the sheep for Diet ML and in the dry cows for Diet MLW. Within livestock class, digestibility was higher on Diet MLW than on Diet M and ML. Table 2 indicates where these differences reached significance. Measures of N metabolism are presented in Table 3 and between-class comparisons were also made on the basis of liveweight to the power 0.90. Intakes of dietary N and digested N and excretion of faecal N were highest for lactating cows on all diets. Excretion of urinary N was similar between classes on Diets M and ML, but was higher in lactating than in dry cows on Diet MLW. Consequently, N balances were highest in lactating cows on all diets. Urine was a more important avenue of N excretion in dry cows and sheep than in lactating cows. Furthermore, urinary N losses were relatively greater than faecal N losses on Diets ML and MLW than on Diet M. Rumen pH did not differ between diets or between classes of livestock; mean values were 6.53, 6.67 and 6.44 for lactating cows, dry cows and sheep,

470

0.691b 0.559a

0.651a 0.551a

385

0.628a

26.1a

43.4a

0.600a

29.7b

52.2b

36.1

0.685b 0.519a

0.630a

26.9ab

44.9a

399

0.645a 0.635a

0.605a

39.8c

70.4c

496

0.659a 0.606a

0.607a

23.5a

40.4a

D

39.5

0.712b 0.676b

0.674b

32.4b

51.7b

S

402

0.708a 0.673a

0.686a

44.7c

69.6c

L

MLW

505

0.783c 0.711b

0.753b

24.2a

33.7a

D

1.0

1.4

x

x

0.751b 0.009 x 0.671a 0.014 x

42.2

D

S

x y

x

y

y

y y

y

z

y

y x

x

x

y

x y

x

x

y

z z

y

x

x

x x

x

x

x

x y

y

y

y

y y

z

x

x

M ML MLW M ML MLW M ML MLW

L

Within-classbetween-diet

0.722b 0.010 x

29.4b

42.8b

S

SE

Within each row within each triplet of livestock class or diet class, values with a different letter are significantly different ( P < 0.05). a, b, c refer to within-diet between-livestock class comparisons; x, y, z refer to within-livestock class between-diet comparisons.

(kg)

Mean live- weight

Digestibility of.' Dry matter Organic matter Nitrogen

(gperkg°'9°perday)

Digestib~organic matter

(gperkg°'9°perday)

Intake o~ Dry matter

L

S

L

D

ML

M

Within-diet between-claze

Intake and digestibility of Diets M (maize silage), M L (maize silage/lucerne) and M L W (maize silage/lucerne/wheat) by lactating cows (L), dry cows (D) and sheep (S)

TABLE 2

C.O

38a

-74

-

40a

-

39a

45a

239

382a 1045c 354a 601c 228a 508a 154a 537c

478a 415a 398b 322a 239a 218a 239a 197a

1646c

736a

867b 737b

-

D

62b

508a 320a 537a -29a

828a

-

57b

808b 390b 520a 288b

1198b

S 778a

D

52a

179

66b

1120c 554a 545c 224a 578b 429a 542b 125a

1665c

L

MLW

59b

696b 342b 502ab 194a

1038b

S

2

36 16 35 36

35

SE

x

x x x x

x

xy

y z y y

y

y

y y y y

y

x

x y x y

x

M

MLW

M

ML

D

L

y

y y z x

x

ML

Within-class between-diet

y

y x y y

x

MLW

x

x x x x

x

M

S

y

z x y y

z

ML

y

y x y xy

y

MLW

(maize silage/lucerne/

Within each row within each triplet of livestock class or diet class, values with a different letter are significantly different ( P < 0 . 0 5 ) . a, b, c refer to within-diet between-livestock class comparisons; x, y, z refer to within-livestock class between-diet comparisons. 1Adjusted for milk nitrogen secretion.

urinary output

Intake Apparently digestible intake Faecal output Urinary output Balance Adjusted balance 1 Urinary output as percent faecalplus

L

S

L

D

ML

M

W i t h i n - d i e t between-class

Nitrogen balance data (ragper kg °9° per day) recorded on Diets M (maize silage), M L (maize silage/lucerne) and M L W wheat) by lactatingcows (L), dry cows (D) and sheep (S)

TABLE 3

CO

98b

(g per kg°'9° per day) Milk output

44a

35a

-

61a

23a

0.88a

39a

69

110b

55b

3.61b

255c

61a

53ab

3.87b

156b

D

62a

40a

1.83a

94a

S

73

120c

52b

3.48b

244b

L

MLW

-

51a

49ab

3.79b

124a

D

-

9

73b

38a 7

4

2.86a 0.21

123a

S

SE

x

x

x

x

M

L

x

y

y

y

ML

y

y

y

y

MLW

x

x

x

x

M

D

x

y

y

z

ML

Within-class between-diet

x

y

y

y

MLW

x

x

x

x

M

S

y

y

y

y

ML

y

y

z

z

MLW

Within each row within each triplet of livestock class or diet class, values with a different letter are significantly different ( P < 0 . 0 5 ) . a, b, c refer to within-diet between-livestock class comparisons; x, y, z refer to within-livestock class between-diet comparisons. ~Water balance = ( feed + liquid) water intake - (urine + faecal) water output.

68

32a

(g per kg° ~ per day)

78b

2.73cl.77b

142c

S

L

D

L

(g per kg°'9° per day) Water balance ~

( l k g -1) Urine output

(g per kg°'9° per day) Water intake per unit dry matter intake

Water intake

ML

M

Within-diet between-class

Water balance data recorded on Diets M (maize silage), ML (maize silage/lucerne) and MLW (maize silage/lucerne/wheat) by lactating cows (L), dry cows (D) and sheep (S)

TABLE 4

f~

306

respectively. R u m e n ammonia-N concentrations were significantly higher on Diets M L W and ML t h a n on Diet M (145 and 134 versus 56 mg N 1- i ) and were significantly higher in sheep and dry cows t h a n in lactating cows (127 and 120 versus 88 mg N 1-1, respectively). Lactating cows drank the most water (see Table 4), but as a proportion of D M intake, there was little difference between lactating and dry cows in water consumed in both feed and liquid form except those on Diet M. Water intake per kg°'9° and per unit D M intake were lower on Diet M t h a n on Diets ML or MLW. Urine output did not differ between lactating and dry cows, but was lowest in sheep. Urine outputs were also lower on Diet M t h a n on Diets ML or MLW. Water balances were highest in lactating cows on all diets, but after TABLE5 Milk production and composition, feed intake and nitrogen metabolism data in lactating cows fed on Diets M (maize silage), ML (maize silage/lucerne) and MLW (maize silage/lucerne/wheat) M Yields (kg day-l) of: Milk Fat-corrected milk Milk fat Protein Solids-not-fat Total solids Content (g kg -~) of: Milk fat Protein Solids-not-fat Total solids Intake (kg day -1) of: Dry matter Neutral detergent fibre Digestible organic matter Nitrogen (g d a y - ~) Intake Faecal output Urinary output Milk secretion Balance (excl. milk secretion) Balance Rumen NH3-N (mg 1-1)

ML 14.5b 16.1a 0.69a 0.41b 1.26b 1.95b

47ab 28b 87b 134a

ll.lb 5.4b 6.3c

15.2ab 17.2a 0.74a 0.41b 1.25b 1.99b

49a 27b 82b 131a

15.4a 7.6a 8.7b

MLW 16.2a 16.9a 0.70a 0.50a 1.58a 2.28a

43b 31a 99a 141a

15.4a 4.6c 9.9a

SE 0.4 0.74 0.04 0.01 0.06 0.08

1 1 3 4

0.3 0.1 0.2

183b 82c 50b 66b

359a 13 l a 111a 65b

364a 119b 127a 79a

8 3 8 2

51b - 15b 21a

117a 52a 130b

119a 40a 113b

10 10 26

Within each row, values with different letter are significantly different (P < 0.05).

307 accounting for water secreted in milk, between-class differences were smaller and generally non-significant. Urinary specific gravity and faecal DM contents were both significantly higher in sheep than in lactating or dry cows and across diets; mean values were 1.035, 1.028 and 1.026 and 326, 171 and 184 g kg -1, respectively. Milk production, feed intake and N metabolism data in lactating cows are presented in Table 5. Intake of digestible OM and yields of milk, protein, solids-not-fat and total solids were highest with Diet MLW. Yields of milk fat and milk corrected to 40 g kg- 1milk fat did not differ between diets. The MLW diet resulted in an increase in contents of protein and solids-not-fat in the milk and a depression in milk fat content. When compared to N balance data while fed Diet ML, the lactating cows had a lower faecal N output and a higher milk secretion of N on Diet MLW, yet similar N balances. DISCUSSION Lactating cows recorded the highest intakes and with one exception, the lowest digestibility of DM, OM and N on each diet. Tyrrell and Moe (1975) and Colucci et al. (1982) both reported lactating cows to have lower digestibility of maize silage-based diets than dry cows. The lower digestibility of these diets in lactating cows than in sheep was recently confirmed at Kyabram when OM digestibility values of 0.700 versus 0.745 were measured with a ration consisting of maize silage:lucerne hay:cottonseed meal:wheat in the ratio 23:20:15:40 (J.B. Moran, unpublished data, 1987). Tyrrell and Moe (1975) considered the higher feed intake and hence faster rate of passage of feed in lactating cows as the major cause of their poor digestibility. Nevertheless, OM digestibility differences between lactating cows and sheep were not observed by Carle and Dulphy (1980) or Nakui et al. (1982) in animals fed ad libitum on maize silage. Digestibility differences between classes of livestock could be confounded with maize silage NDF contents, particularly when animals are fed to appetite, as van Soest (1982) noted that increases in intake had a greater negative effect on digestibility in feeds with higher NDF contents. Reports on the comparative digestibility in dry cows and sheep are also confusing as Aerts et al. (1985) reported similar OM digestibility values for 26 different maize silages, whereas Deoka et al. (1986) reported higher DM and N digestibility in the dry cows when fed both the components of and a total mixed ration based on maize silage. There were differences between the livestock classes in their ranking of the 3 diets on the basis of voluntary feed intake. Dry cows and sheep ate more of Diet M than of Diet MLW whereas the reverse applied with lactating cows. This suggests that for animals not subjected to the energy stress of late pregnancy or lactation {in this case dry cows or sheep), appetite regulation maybe more influenced by dietary energy concentration than is the case with lactating

308

animals. This precludes the ranking of diets based on voluntary feed intake for animals other than the class of livestock or the physiological state for which they were intended. Lactating cows and sheep gave the same ranking of diets on digestibility, but this was different from that given by dry cows. In attempts to predict the nutritive value of mixed diets for cattle, Tyrrell and Moe (1975) commented that the digestibility of specific feeds was not constant but changed with intake, the rate of change being more or less dependent upon the combination of ration ingredients. Changes in dietary ME would be smaller because of the compensatory effects of lower energy losses from methane and urine at higher feed intakes (van Es, 1978). The lack of response of lactating and dry cows to additional lucerne suggests that in this experiment, protein may not be limiting digestibility. That sheep did respond to lucerne, may reflect their superior ability to select a higher quality diet when compared to that of cattle. The improvements in digestibility in all classes when wheat was given reflects both the high proportion of wheat in the diet and its greater digestibility than the other components. A synergistic effect on digestibility of the additional energy (wheat) and protein (lucerne) cannot be discounted. The fact that improvements were greater in dry cows than in sheep needs further investigation. These conclusions have particular relevance to studies where effect of intake and class of livestock are confounded (e.g., Schiemann et al., 1971; E1 Khidir and Thornsen, 1983). The similarity in ranking of diets on the basis of digestibility by lactating cows and sheep supports the conclusions of van Es (1978) and Lindgren (1981). Both authors present generalized equations for predicting ME of mixed diets for lactating cows using digestibility data derived from sheep fed at maintenance. Differences between livestock classes in both feed intake and digestion in the present trial would be partly related to differences in dietary N content. The inclusion of urea in these diets was at the maximum level recommended for dairy production rations (Bath et al., 1978), consequently Diet M contained only 17.8 whereas Diets ML and MLW contained 23.6 and 25.1 g N kg- 1DM, respectively. Diet M was then below recommended protein levels for lactating cows whereas Diets ML and MLW contained sufficient protein for the production of up to 18 kg milk day-1 (National Research Council, 1978). Oldham (1984) noted that an increase of from 17 to 24 g N kg -1 DM had no effect on digestibility in dry cows, but had a big effect in lactating cows. Furthermore, he noted that increases in dietary N contents usually led to increases in ad libitum feed intake. In fact, he calculated average responses due to protein on maize silage-based diets, which in this case would have increased DM digestibility by 4 units and DM intake by 1.5 kg day- 1 when cows were changed from Diet M to Diet ML. The observed increases were 0.5 DM digestibility units and 4.3 kg day- ~ DM intake. Therefore, in these cows, the stimulation

309

to intake when offered Diet ML was due to dietary ingredients other than N, while with DM digestibility, the additional lucerne in Diet ML may have reduced the magnitude of the protein effect suggested by Oldham (1984). In the dry cows, the additional lucerne in Diet ML reduced DM digestibility by 2.1 units and had little influence on DM intake, whereas in the sheep, the lucerne increased DM digestibility by 4.4 units and also stimulated appetite. The protein effect of changing from Diet ML to MLW would have been overshadowed by any benefits arising from the inclusion of 0.50 of a readily digested carbohydrate source in the diet. There appear to be differences between classes of livestock in their ability to physically process maize grain from silage. Comparing sheep and steers, Deinum et al. (1984) found poorer mastication in steers which led to an excretion of 0.03 of the OM intake of maize silage in the form of whole maize grains; digestibility values for sheep and steers were 0.752 and 0.727, respectively, but when corrected for whole grain excretion, the value for steers increased to 0.743. Carle and Dulphy (1980) and Nakui et al. (1982) both reported poorer starch digestion in lactating cows than in sheep fed on maize silage, while the former authors recorded faecal excretion in the cows of 0.10 of the ingested maize grains; this is similar to the 0.09 observed by J.B. Moran (unpublished data, 1986). The poorer digestion of maize starch in cattle is also the result of a greater escape of the starch from the tureen compared to that in sheep (0.30 versus 0.11; Theurer, 1986). Colovos et al. (1970) found that sheep and steers utilized maize silage to a similar extent when harvested at the soft dough and the medium to hard dough stages (with DM contents of 240 and 260 g kg -1, respectively), but when harvested at the early dent (300 g kg -1 DM) or glazed and frosted (390 g kg -~ DM) stage, net energy values were lower in the steers. It then appears that although sheep may be used to predict dietary quality of maize silage-based diets for dairy cows, an interaction between animal class and crop maturity cannot be discounted. For all diets, lactating cows had the highest N balances, which reflected their higher intake of N. Compared to dry cows and sheep, the lactating cows lost less of their digested N through urine and this would be expected considering the high levels of recycling of urea as part of the N conservation mechanisms operating during early lactation (Oldham, 1984). Van Es (1978) calculated that some 0.30 of the digested N was converted to milk N at milk production levels between 15 and 20 kg day -~. Corresponding values in the present study were 0.65, 0.29 and 0.32 for Diets M, ML and MLW, respectively (Table 5). Together with the observed N balances, these data suggest that tissue N was contributing to milk N only on Diet M. Diets ML and MLW apparently supplied sufficient ME and N to satisfy the needs of maintenance and milk production. With the dry cows, urinary N losses were highest and N balance was lowest on Diet ML, suggesting that the addition of wheat not only improved digestibility of dietary N, but also its assimilation into body tissues. The inclu-

310 sion of wheat in the diet of the sheep, on the other hand, had little effect on faecal or urinary losses of N. The mos~ striking difference in the water metabolism of the 3 classes of livestock was the ability of the sheep to extract more water from both faeces and urine. Sheep had the lowest water intake (per kg°'9° and per unit DM intake), the lowest urine output and the highest water balance (the latter after accounting for milk production in the lactating cows). The environmental temperatures during this study would be unlikely to have different effects on losses of water through cutaneous and respiratory avenues in sheep and dairy cows (National Research Council, 1981 ). Differences between diets in intakes of digestible OM by the lactating cows were reflected in corresponding differences in milk yield and production of milk protein, solids-not-fat and total solids (Thomas, 1984). Yields of fatcorrected milk were influenced by diet to a lesser degree because of the decrease in milk fat content with Diet MLW which would have resulted from the lower intake of NDF on that diet. Sutton (1984) noted that milk fat contents decreased once dietary ADF levels fell below 200 g kg- 1 DM and ADF levels fell from 351 in Diet ML to 189 g kg -1 DM in Diet MLW in the present trial. The low rumen ammonia concentration on Diet M is at the level at which utilization of the added urea in the diet would be almost complete; however, the levels on Diets ML and MLW are above 50 mg N 1-1, the level suggested by Satter and Roffler (1981) to be the upper limit for utilization of added non-protein N. Therefore, despite the inclusion of 0.50 wheat in Diet MLW, the urea would largely be excreted as urinary urea N. In fact with such a diet, 19.2 g N kg-1 of DM may be the upper limit for utilization of added non-protein N (Satter and Roffler, 1981). With the van Es (1978) prediction equation for maize silage (ME = 15.5 × digestible OM concentration), dietary ME contents of M, ML and MLW can be estimated to be 8.8, 8.8 and 10.0 MJ kg-1 DM, respectively, in lactating cows. In 6 years of evaluating maize silage at Kyabram, estimates of ME have not exceeded 10.4 MJ kg-1 DM (Moran, 1986), whereas ME values of as high as 11.9 and 11.8 MJ kg - 1DM have been reported in Holland (Deinum et al., 1984) and the U.K. (Wainman et al., 1978), respectively. The corresponding NDF contents were 460 (Kyabram), 332 (The Netherlands) and 382 g kg- I DM (U.K.). The high summer temperatures and solar radiation at Kyabrain during crop growth and grain maturation lead to very high yields of irrigated forage maize, e.g., 31 tonne DM ha -1 (Pritchard, 1987). However, such high DM accumulation in forage maize is invariably associated with increases in contents of plant NDF and lignin (Struik, 1983). This would then reduce intake and ME value of the ensiled crop, thus leading to lower milk yields when compared to those obtained in other temperate countries.

311 ACKOWLEDGEMENTS

The authors wish to thank C. Connally and the Kyabram fieldstafffor animal management and S, Soares for laboratory technical support.

REFERENCES

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