The effect of cutting time of grass silage in primary growth and regrowth and the interactions between silage quality and concentrate level on milk production of dairy cows

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Livestock Science 116 (2008) 171 – 182 www.elsevier.com/locate/livsci

The effect of cutting time of grass silage in primary growth and regrowth and the interactions between silage quality and concentrate level on milk production of dairy cows K. Kuoppala a , M. Rinne a,⁎, J. Nousiainen b , P. Huhtanen a,1 a b

MTT Agrifood Research Finland, FI-31600 Jokioinen, Finland Valio Ltd, Farm Services, P.O. Box 10, FI-00039 Valio, Finland

Received 12 December 2006; received in revised form 25 September 2007; accepted 1 October 2007

Abstract Two silages were prepared from the primary growth (PG) of timothy-meadow fescue sward at early (E) and late (L) stage of growth. The subsequent regrowth (RG) areas were further harvested at early (EE and LE) and late (EL and LL) stages of growth resulting in six silages in total. The silages were fed ad libitum to 24 lactating Finnish Ayrshire cows and supplemented with 8 or 12 kg concentrate per day in a cyclic change-over experiment with four 21-day periods and 6 × 2 factorial arrangement of treatments. The quality of silages varied markedly within and between the harvests although variation was greater within PG than RG. Postponing the harvest in PG decreased silage dry matter (DM) intake by 0.48 kg and energy corrected milk yield (ECM) by 0.61 per 10 g decrease in silage D-value (concentration of digestible organic matter in DM), while responses and the range between the diets were clearly smaller when RG silages were fed. On average, ECM yield was higher when PG rather than RG silages were fed. The mean response to increased concentrate DM intake was 0.62 kg ECM using diets based on PG. The response increased with increasing growth stage of grass being 0.34 and 1.01 kg ECM / kg additional concentrate DM for E and L, respectively. The difference was mainly mediated by the differences in substitution rates (reduction in silage DM intake per increase in concentrate DM intake, kg/kg), which were 0.71 and 0.22 for E and L, respectively. The ECM response to increased concentrate allowance was on average greater when RG rather than PG silages (0.92 vs. 0.62 kg/kg concentrate) were fed. Milk production of dairy cows reflected the intake of metabolizable energy and no differences in the utilization of it were found between diets based on silages harvested from PG and RG. However, intake of RG silages was slightly lower than that of comparable PG silage, but definite reasons for that could not be identified. © 2007 Elsevier B.V. All rights reserved. Keywords: Forage; Harvesting strategy; Digestibility; Intake; Grass maturity

1. Introduction ⁎ Corresponding author. Tel.: +358 3 4188 3660; fax: +358 3 4188 3661. E-mail address: [email protected] (M. Rinne). 1 Current address: Cornell University, Department of Animal Science, 269 Morrison Hall, Ithaca, NY 14853-4801, USA. 1871-1413/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.livsci.2007.10.002

Cutting time of grass in first cut has proven to be a major factor affecting forage digestibility and subsequently intake and milk production of dairy cows. The average dry matter (DM) intake and milk yield responses were 0.16 and 0.32 kg/d per 10 g increase in silage D-value (digestible

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organic matter, g/kg DM) in the literature review of Rinne (2000). During the first growth of grass in spring, i.e. the primary growth, the daily decline in D-value has typically been 5 g/kg DM (Rinne et al., 1999, 2002; Dawson et al., 2002). Most studies investigating grass maturity effects on feed intake and milk production have been conducted using silages made from primary growth of grass. However, large amounts of regrowth grass silages are also used for dairy cows. Regrowth grass is typically leafier (Beever et al., 2000; Rinne and Nykänen, 2000; Kuoppala et al., 2003; Gustavsson and Martinsson, 2004), contains less cell wall carbohydrates (neutral detergent fibre, NDF) but simultaneously is less digestible (Huhtanen et al., 2006) than the corresponding primary growth grass. Lower digestibility of regrowth grass is related to the higher proportion of indigestible NDF (iNDF) in the cell wall compared with the material harvested from the primary growth (Huhtanen et al., 2006). There are only a few published experiments where comparisons of milk production potential between primary growth and regrowth grass silages have systematically been conducted. Further, in these studies the season (primary growth vs. regrowth) effects have often been confounded by differences in digestibility and silage fermentation characteristics. Castle and Watson (1970) and Khalili et al. (2005) observed that silage intake and milk production were higher with silages made from the primary growth than from the regrowth of grass, but in both cases, regrowth silages were lower in digestibility. However, Peoples and Gordon (1989) and Heikkilä et al. (1998) used regrowth grass silages of similar or higher digestibility than in primary growth and yet silage intake and milk production were higher when silage made from primary growth was fed. To estimate the season effects on milk production potential, the primary growth and regrowth silages should be harvested at equal quality. In practise, this is difficult to achieve. By harvesting both the primary growth and regrowth silages at different growth stages, it is possible to produce silage qualities which overlap between the harvests. Such data would allow unbiased comparisons to be made between the harvests. Our main objective was to compare the milk production potential of primary growth and regrowth grass silages. The primary growth silages were harvested at two contrasting growth stages and the subsequent regrowth silages from both primary growth harvest areas were also harvested at two growth stages. The second objective was to investigate the possible interactions between silage quality and amount of concentrate supplementation. Although the general responses to concentrate supple-

mentation on forage-based diets are well documented (Thomas, 1987; Huhtanen, 1998), the magnitude of responses with varying silage quality requires further study. A preliminary report of the present results has been presented by Kuoppala et al. (2005). 2. Materials and methods 2.1. Production of the experimental silages Two primary growth (PG) and four regrowth (RG) silages were made from a mixed timothy (Phleum pratense cv. Iki) meadow fescue (Festuca pratensis cv. Antti) sward in 2002 at the experimental farm of MTT Agrifood Research Finland, Jokioinen (61°49′N, 23°28′E). Two similar fields (11.9 and 7.9 ha) on clay soils with swards established in 1999 were divided into two areas for PG harvest. Commercial N–P–K fertilisers were applied at rates of 96–7–11 kg/ha for PG and 78–0–3 kg/ha for RG. The PG silages were harvested on 5 June at early (E) and on 17 June at late (L) stage of growth. The RG silages were harvested from the regrowth areas E and L on 29 July at early (EE and LE) and on 12 August at late (EL and LL) stage of growth. The grass was cut with a mower-conditioner, wilted for approximately 4 h, harvested with a precision-chop harvester, and preserved with a formic-acid based additive (760 g formic acid and 55 g ammonium–formiate per kg) applied at a rate of 5.4 l/tonne into bunker silos of 70 t capacity. Minimum wilting and high level of formic-acid application were used to minimize the confounding effects of DM concentration and silage fermentation characteristics on the comparison of growth stage and season effects. Each feed wagon was weighed and sampled to measure DM yield and to obtain representative samples of the grass ensiled. The exact acreage of each area was measured using GPS device (Trimble ProXR, Geotrim Ltd, Finland) to calculate the yield per hectare. For botanical and morphological analysis, samples of the stands were collected from both fields from separate 20 × 5 m areas adjacent to the silage harvest areas. Samples were collected with 0.5 × 0.5 m frames by cutting at least 4 frames from both fields. The height of the standing herbage was measured from the 3 sides of each frame. The samples of the two fields were analysed separately, but a mean of both fields is presented. Morphological analysis was conducted by manually dissecting the plants into leaves, stems and inflorescences. Proportions of timothy, meadow fescue, other species and dead material in the sample DM were determined. Weather data was obtained from the Finnish Meteorological Institute. The cumulative temperature was calculated from the onset of growth as Σ (daily mean temperature — 5 °C) and expressed as degree-days (°D). 2.2. Diets and design of the experiment Dietary treatments in a 6 × 2 factorial arrangement consisted of the six experimental silages and two concentrate levels

K. Kuoppala et al. / Livestock Science 116 (2008) 171–182

(8 and 12 kg/d on fresh weight basis). Concentrate supplement comprised of (g/kg fresh matter) barley (300), wheat bran (162), molassed sugar-beet pulp (150), rapeseed meal (130), wheat meal (63), oats (50), wheat molasses (50), soybean meal (20), maize gluten meal (23), rapeseed oil (10) and minerals and vitamins (42). The diets were fed to 24 Finnish Ayrshire cows 68 (S.D. 23.9) days in lactation and 624 (S.D. 64.0) kg live weight (LW) in the beginning of the experiment. The experiment was conducted as a cyclic change-over design (Davis and Hall 1969) with four 21-day periods using two blocks of twelve cows each. The cows were assigned into the blocks according to milk yield [mean 41.6 (S.D. 1.84) and 34.7 (S.D. 1.73) kg/d for blocks 1 and 2, respectively] and allocated at random to the experimental treatments. Cows were housed in individual stalls with a 20 h access to feed and a free access to drinking water. They were milked twice a day at approximately 6:30 and 15:30 h. Concentrates were offered in three equal meals daily at 5:30, 13:00 and 19:00 h. Silage was given twice daily at 5:30 and 12:30 and added as necessary to ensure proportionate refusals between 5 and 10% of total silage intake.

were taken on two milkings. Concentration of urea was calculated from a difference in concentration of ammonia N between unhydrolysed sample and that of hydrolysed with urease. Ammonia N was determined by the method of McCullough (1967). Whole tract digestibility of the diets was estimated with the 12 cows in the higher yielding block by using AIA as an internal marker (Van Keulen and Young, 1977). During the last five days of each period, faecal spot samples of approximately 200 g were collected twice daily at 7:00 and 15:00 h. At the end of each period the samples were pooled on an individual cow basis, thoroughly mixed, subsampled and stored at − 20 °C. Cows were weighed at the beginning of the experiment and at the two last days of each experimental period at 10:00 h. The LW change was calculated by regression.

Table 1 Description of the herbage ensiled at different stages in first growth of the season and in the following regrowth Primary growth

2.3. Experimental procedures and chemical analyses Food intake and milk yield were recorded daily. The last 7 d of each period were used to calculate the results. Representative samples of silage, concentrates and feed refusals (if exceeded 0.3 kg/d) were collected daily during the sampling period and stored at −20 °C. At the end of the experiment the samples were thawed, mixed and submitted for chemical analysis. Fresh silage samples were analysed for pH, concentration of volatile fatty acids (Huhtanen et al., 1998), lactic acid (Haacker et al., 1983), water-soluble carbohydrates (WSC; Somogyi, 1945), ammonia N (McCullough, 1967) and watersoluble N. The DM concentration was determined by drying at 105 °C for 20 h and organic matter (OM) concentration by ashing at 600 °C for 2 h. Oven DM concentration of silages was corrected for the loss of volatiles according to Huida et al. (1986). Concentration of NDF was determined according to Van Soest et al. (1991) using Na-sulphite, without amylase for forages and presented ash-free. Acid detergent fibre and permanganate lignin were determined according to Robertson and Van Soest (1981). Crude protein (CP) concentration was analysed by the Dumas method using Leco FP 428 nitrogen analyser (Leco Corp., St Joseph; USA). Concentration of acid insoluble ash (AIA) was determined following sequential acid hydrolysis (Van Keulen and Young, 1977). The iNDF concentration was determined by 12 d ruminal incubation in the rumen of dairy cows fed a forage-based diet and using nylon bags with a pore size of 17 μm (Huhtanen et al., 1994) and expressed ash-free. Potentially digestible NDF (pdNDF) was calculated as NDF−iNDF. Milk samples were taken on four consecutive milkings during the last week of each period and analysed by an infra-red analyser (Milko-Scan 605, Foss Electric, Hillerød, Denmark) for fat, protein and lactose. Milk samples for urea determination

173

Stage of growth

Early (E)

Late (L)

Regrowth of E

Regrowth of L

Early Late

Early Late

Date of harvest in 5 17 29 12 2002 June June July August Date of previous – – 5 5 harvest June June 46 58 54 68 Growing time a (d) Degree days a 297 431 644 825 (°D) Mean temperature a 11.5 12.5 17.0 17.2 (°C/d) Proportion of herbage [g/kg dry matter (DM)] Timothy 815 822 688 671 Meadow fescue 163 155 253 209 Other species b 22 23 59 120 Dead material 9 12 81 101 Proportion of leaves within species (g/kg DM) Timothy 489 340 503 390 Meadow fescue 538 491 769 711 Height of the 42 75 67 78 sward, cm DM yield (kg/ha) 3300 5100 4200 4900 Dry matter (g/kg) 270 278 224 319 Chemical composition (g/kg DM) Ash 74 62 87 88 Crude protein 151 115 129 118 NDF 513 598 566 562 ADF 243 298 304 308 Lignin 21 27 21 29 Indigestible NDF 45 90 66 83 a

29 July 17 June 42

12 August 17 June 56

507

688

17.2

17.4

592 210 199 60

544 226 230 116

639 794 54

529 801 71

3100 3800 209 308 91 153 549 289 22 52

84 126 549 292 21 70

Calculated in primary growth from the beginning of growing season on 20 April 2002 and in regrowth from the previous harvest. b Mainly couch grass (Elymus repens) and dandelion (Taraxacum officinales).

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Table 2 Description of the experimental silages and the concentrate supplement Primary growth

Regrowth of E

Regrowth of L

Concentrate

Stage of growth

Early (E)

Late (L)

Early

Late

Early

Late

Dry matter (DM; g/kg) Chemical composition (g/kg DM) Ash Crude protein NDF ADF Lignin Indigestible NDF Lactic acid Acetic acid Propionic acid Butyric acid WSC a Ammonium N (g/kg N) Soluble N (g/kg N) pH OMD b (g/kg) D-value c (g/kg DM) ME d (MJ/kg DM) AAT e (g/kg DM)

274

283

235

323

230

320

862

83 160 498 261 24 50 68.4 17.1 0.62 1.55 34.0 47 614 4.11 767 704 11.3 86.9

68 127 594 317 31 97 36.9 12.0 0.20 0.94 94.1 61 607 4.23 692 644 10.3 79.2

97 146 534 313 26 71 60.8 16.9 0.08 0.65 28.6 60 491 4.07 727 659 10.5 81.8

92 115 541 310 29 93 31.3 10.0 0.09 0.58 127.3 70 512 4.45 673 609 9.7 75.1

97 159 518 304 25 60 67.5 17.5 0.22 1.16 26.0 58 522 4.1 734 664 10.6 83.1

86 130 530 302 28 79 28.2 9.5 0.08 0.19 126.1 62 499 4.47 689 629 10.1 78.0

80 179 256 110 16 51

a b c d e

12.4 109.2

Water-soluble carbohydrates. Organic matter digestibility determined in vivo using sheep. Digestible organic matter in DM determined in vivo using sheep. Metabolizable energy determined in vivo using sheep. Amino acids absorbed in the small intestine.

Apparent in vivo digestibility of the six silages was measured simultaneously with the milk production experiment with sheep by total faecal collection. The digestibility trial was conducted according to an incomplete Latin square design (six silages, six sheep and four periods) with 21-day periods the last seven days being used for faecal collection. Silages were offered at approximately maintenance level (40 g DM per kg LW0.75) and supplemented daily with 30 g mineral mixture and 10 g NaCl. Sheep were fed twice daily and had a free access to drinking water.

the feeds were calculated according to the Finnish feed tables (MTT, 2006). Milk energy concentration and subsequently energy corrected milk (ECM) yield was calculated using the formula of Sjaunja et al. (1990). Utilization of AAT was calculated as milk protein output / (AAT intake − AAT for maintenance) and ME utilization as milk energy output / (ME intake − ME for maintenance). The effects of LW change were ignored and maintenance requirements defined by MTT (2006) were used. 2.5. Statistical analysis

2.4. Calculations The metabolizable energy (ME) concentration of the silages was calculated from the in vivo (sheep) D-value × 0.016 (MAFF, 1975). The ME concentration of the concentrates was calculated from digestible nutrients (MAFF, 1975) using digestibility coefficients reported by MTT (2006). The ME derived from these calculations is called MEFT. The intake of ME of the cows was also estimated from the intake of digestible organic matter (DOM) measured by AIA assuming a ME concentration of 16 MJ/kg DOM (MEDOM). Apparent digestibility of OM measured individually with AIA was used for cows in the higher yielding block and the diet mean for cows in the lower yielding block. The amino acids absorbed in the small intestines (AAT) representing the metabolizable protein concentration and the protein balance in the rumen of

Experimental data was subjected to analysis of variance using the general linear model procedure (PROC GLM) of the Statistical Analysis Systems Institute (SAS®, 2003). For intake and milk production data, the model included the effects of block, cow within block, period, treatment, treatment carryover and interactions of block × diet and block × period. Digestibility data was analysed with the same model with the exception that the effects of block and treatment carry-over were omitted. Sums of squares of the treatment effects were further separated using contrasts: C1 = effect of concentrate level, C2 = effect of growth stage in PG, C3 = PG vs. RG, C4 = effect of harvest time of PG in regrowth (EE&EL vs. LE&LL) and C5 = effect of growth stage in regrowth (EE&LE vs. EL&LL). Further contrasts were performed to assess the significance of interactions between the main treatment effects.

Table 3 The effects of harvesting strategy and concentrate level on daily feed and nutrient intake of dairy cows Growth stage

Primary growth

8

Late (L) 12

8

Early 12

8

Late 12

Statistical significance a

Regrowth of L

8

Early 12

8

Late 12

8

12

SEM b

Feed and nutrient intake (kg/d) Silage dry matter (DM) 17.4 15.0 13.8 12.9 13.0 11.4 12.9 11.7 13.7 11.9 13.7 11.8 0.18 Concentrate DM 6.9 10.2 6.9 10.3 6.9 10.3 6.9 10.3 6.9 10.3 6.9 10.3 0.01 Total DM 24.3 25.3 20.7 23.2 19.9 21.7 19.7 22.0 20.6 22.3 20.6 22.2 0.18 Organic matter 22.2 23.2 19.2 21.5 18.1 19.7 18.0 20.1 18.7 20.3 18.9 20.3 0.17 Crude protein 4.02 4.23 2.97 3.48 3.13 3.50 2.72 3.21 3.38 3.70 3.02 3.38 0.028 NDF 10.4 10.1 10.0 10.3 8.7 8.7 8.7 8.9 8.9 8.8 9.0 8.9 0.10 Indigestible NDF 1.21 1.28 1.70 1.77 1.25 1.33 1.54 1.60 1.16 1.24 1.43 1.46 0.021 Potentially digestible NDF 9.19 8.84 8.27 8.54 7.42 7.36 7.14 7.32 7.69 7.56 7.56 7.44 0.085 MEDOM c (MJ/d) 261 264 205 226 207 231 197 223 219 237 214 230 2.68 281 296 227 261 223 247 210 242 231 255 224 247 1.83 MEFT d (MJ/d) AAT e (g/d) 2268 2430 1840 2143 1803 2044 1734 2019 1898 2122 1837 2060 15.1 PBV f (g/d) 195 165 − 100 − 83 118 105 −186 − 155 229 197 − 52 −54 12.8

C1

C2

C4

C5

⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎

ns

⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ns ⁎⁎⁎ ns ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ns

ns ns ⁎⁎⁎ ns ⁎⁎⁎ ⁎ ⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎

⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ns ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎

C3

⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎

⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ns ⁎⁎⁎ ⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎

a Orthogonal contrasts: C1 = effect of concentrate level; C2 = effect of growth stage in primary growth; C3 = primary growth vs. regrowth; C4 = effect of harvest time of primary growth to the regrowth (EE&EL vs. LE&LL); C5 = effect of growth stage in regrowth (EE&LE vs. EL&LL). b SEM: Standard error of the mean (n = 8). c MEDOM: ME intake was calculated from the intake of digestible organic matter (DOM) determined with AIA, assuming ME concentration of 16 MJ/kg DOM. d MEFT: ME content of feeds was calculated using digestibility coefficients determined in vivo with weathers for silages and feed table values for concentrates. e Amino acids absorbed from the small intestine. f Protein balance in the rumen.

K. Kuoppala et al. / Livestock Science 116 (2008) 171–182

Early (E) Concentrate level

Regrowth of E

175

176

K. Kuoppala et al. / Livestock Science 116 (2008) 171–182

of iNDF / daily increase of NDF; g/kg DM) was much greater in RG than in PG (2.14 vs. 0.48, respectively). The fermentation quality of all silages was good as evidenced by low pH (mean 4.24) and concentration of ammonia N (mean 60 g/kg N; Table 2). In-silo fermentation was more extensive in early harvested silages compared to the late harvested silages as evidenced by lower pH and WSC concentration and higher lactic acid and volatile fatty acid concentrations.

3. Results 3.1. Herbage and silage characteristics The herbage characteristics varied markedly within and between the harvests although variation was greater within PG than RG (Table 1). Mean daily temperature and accumulated degree-days were higher during RG than PG. The proportion of timothy was higher in PG than in RG (on average 819 vs. 624 g/kg DM), but the proportions of other plant species and dead material were higher in RG than PG. The proportion of leaves in grass DM was higher in meadow fescue than in timothy, in RG than in PG and in early harvested than in late harvested herbages. The DM concentrations of E and L were on the same level, while in RG the early harvested grasses had lower DM concentration than late harvested grasses reflecting the weather conditions prior to the harvest dates. The accumulation of herbage DM per hectare was clearly faster in PG (150 kg/d) than in RG (50 kg/d). The DM concentration of the silages reflected that of the parent herbages (Table 2). Primary growth silages had a lower mean ash concentration than RG silages (76 vs. 93 g/kg DM), which results in a higher D-value at the same OMD. Comparisons of forage digestibility are however conducted on D-value basis, because ash contributes no energy to the animal. The RG silages were on average less digestible than the PG silages the mean D-values being 674 and 640 g/kg DM, respectively. Delayed harvest in PG decreased D-value by 5.0 g/kg DM per day, while the corresponding changes in RG were 3.6 g/kg DM from EE to EL and 2.5 g/kg DM from LE to LL, respectively. The relationships between chemical composition and D-value differed in PG and RG as in PG, similar D-value was achieved at a higher NDF concentration. The rate of iNDF accumulation in relation to NDF accumulation (daily increase

3.2. Feed and nutrient intake and diet digestibility Postponing the harvest in PG decreased silage DMI by 0.48 kg per 10 g decrease in silage D-value, and subsequently decreased intake of total DM and other nutrients (P b 0.001) except NDF (Table 3). The silage and total DM and nutrient intakes were greater on PG rather than RG based diets (P b 0.001). The carry-over effect from harvest time of PG (EE&EL vs. LE&LL) significantly affected the intakes of silage and total DM and nutrient intakes (P b 0.05) except NDF, while the effect of progressing regrowth (EE&LE vs. EL&LL) was not significant on silage and total DMI, although intakes of ME and AAT increased (P b 0.01). Postponing the harvest in PG decreased significantly the apparent digestibility of DM, OM, NDF and pdNDF (Table 4). Digestibility of DM (P b 0.05), OM, NDF and pdNDF (P N 0.01) were on average higher in RG than PG diets, but no difference in CP digestibility was found. Differences in digestibility were also found within RG as both the carry-over effect from harvest time of PG and the progressing regrowth significantly decreased NDF and pdNDF digestibility. The early harvested RG silages were also higher in DM, OM and CP digestibility (P b 0.01), while the time of harvest in first growth did not significantly affect RG silage digestibility.

Table 4 The effects of harvesting strategy and concentrate level on apparent whole tract digestibility of dairy cows Growth stage

Primary growth

Regrowth of E

Early (E)

Late (L)

Early

8

8

Concentrate level 8 Dry matter Organic matter Crude protein NDF pdNDF b a b

0.724 0.737 0.687 0.665 0.754

12 0.694 0.709 0.663 0.596 0.681

See Table 3. Potentially digestible NDF.

0.654 0.668 0.647 0.572 0.689

12 0.643 0.658 0.636 0.520 0.629

0.700 0.720 0.685 0.650 0.760

Late 12 0.702 0.725 0.684 0.622 0.732

Statistical significance a

Regrowth of L

8 0.666 0.684 0.620 0.582 0.708

Early 12 0.676 0.696 0.627 0.551 0.672

8 0.713 0.732 0.688 0.680 0.782

Late 12 0.712 0.733 0.688 0.642 0.746

8 0.685 0.704 0.640 0.625 0.744

12

SEM a

0.691 0.710 0.653 0.586 0.701

0.0091 ns ⁎⁎⁎ 0.0089 ns ⁎⁎⁎ 0.0106 ns ⁎ 0.0125 ⁎⁎⁎ ⁎⁎⁎ 0.0086 ⁎⁎⁎ ⁎⁎

C1

C2

C3

C4 C5

⁎ ⁎⁎ ns ⁎⁎ ⁎⁎⁎

o o ns ⁎⁎ o

⁎⁎ ⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎

K. Kuoppala et al. / Livestock Science 116 (2008) 171–182

177

affected while protein (P b 0.001) and lactose (P b 0.05) concentrations were lower when L rather than E was fed. Milk, ECM, fat, protein and lactose yields and milk fat and protein concentrations were lower (P b 0.001) in RG than PG diets while lactose concentration was not affected and milk urea concentration was higher (P b 0.05) when RG silages were fed. Cows consuming RG silages of L had higher yields of milk, ECM (P b 0.01), fat (P b 0.05), protein and lactose (P b 0.01), and higher concentrations of lactose (P b 0.05) and urea (P b 0.001) than those consuming RG silages of E. With progressing regrowth (EE&LE vs. EL&LL) the effects were similar, but smaller in magnitude. Postponing the harvest in PG increased the apparent efficiency of utilization of AAT, MEDOM and MEFT (P b 0.001). Efficiencies of AAT (P b 0.05) and MEFT (P b 0.001) utilization were higher in diets based on RG rather than PG silages, but no significant difference between the seasons was found in the utilization of

Increase in concentrate allowance decreased (P b 0.001) silage DMI. The substitution rate (decrease in silage DM intake when concentrate DM intake is increased, kg/kg) was on average 0.47. However, total DM intake and intakes of nutrients other than NDF and pdNDF increased (P b 0.001) in response to increased concentrate allowance. Increasing the level of concentrate did not affect the digestibility of DM, OM or CP, but decreased (P b 0.001) the digestibility of NDF. An interaction was found in OMD between the level of concentrate and harvest: OMD decreased on PG diets, but increased on RG diets in response to increased concentrate supplementation (P b 0.05). 3.3. Milk production and efficiency of nutrient utilization Postponing the harvest in PG decreased milk (P b 0.01), ECM, fat, protein (P b 0.001) and lactose (P b 0.05) yields (Table 5). Milk fat and urea concentrations were not

Table 5 The effects of harvesting strategy and concentrate level on milk production, it's efficiency and live weight (LW) of dairy cows Growth stage

Primary growth Early (E)

Concentrate level 8 Production per day Milk (kg) 32.2 ECM b (kg) 35.4 Fat yield (g) 1517 Protein yield (g) 1104 Lactose (g) 1557 Milk composition Fat (g/kg) 47.2 Protein (g/kg) 34.3 Lactose (g/kg) 48.5 Urea (mmol/l) 4.20 Efficiency of utilization 0.273 Nc AAT d 0.660 MEDOM e 0.574 MEFT f 0.518 1.46 DM g) Live weight (kg) 637 LW change (kg/d) 0.78 a

Regrowth of E

Statistical significance a

Regrowth of L

Late (L)

Early

12

8

12

8

12

8

Late 12

8

Early 12

8

Late 12

SEM a C1

33.8 36.6 1524 1179 1645

28.9 30.9 1313 939 1418

32.4 34.0 1400 1079 1594

28.1 29.3 1219 917 1356

30.5 31.9 1322 1005 1479

27.2 28.0 1161 874 1321

30.6 31.4 1286 992 1490

30.3 31.0 1273 981 1466

32.9 34.0 1384 1081 1623

28.0 29.1 1208 901 1367

31.6 32.7 1330 1045 1560

0.53 0.54 25.5 18.4 30.0

45.3 35.1 48.6 4.27

45.7 33.0 49.1 3.82

43.3 33.7 49.2 4.43

43.6 33.0 48.2 4.97

43.7 33.3 48.4 4.67

42.7 32.2 48.6 3.10

42.1 32.7 48.8 3.70

42.0 32.6 48.4 5.22

42.4 33.1 49.3 5.15

43.4 32.4 48.8 3.97

41.9 33.1 49.4 4.05

0.310 0.647 0.583 0.502 1.45 636 0.92

0.305 0.713 0.697 0.603 1.50 625 − 0.51

0.289 0.687 0.675 0.550 1.46 638 0.42

0.283 0.708 0.650 0.587 1.47 626 − 0.45

0.317 0.669 0.608 0.553 1.47 631 0.28

0.306 0.711 0.675 0.611 1.43 628 −0.13

0.285 0.676 0.628 0.564 1.43 630 0.41

0.287 0.712 0.636 0.590 1.51 625 − 0.28

0.294 0.690 0.624 0.565 1.53 627 0.24

0.303 0.687 0.622 0.584 1.41 628 0.02

0.273 0.690 0.622 0.566 1.47 631 0.06

C2

C3

C4

C5

⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎

⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎

⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎

⁎⁎ ⁎⁎ ⁎ ⁎⁎ ⁎⁎

⁎ ⁎ ⁎ ⁎⁎ o

0.68 0.26 0.20 0.108

⁎ ⁎⁎ ⁎ ⁎

o ⁎⁎⁎ ⁎ ns

⁎⁎⁎ ⁎⁎⁎ ns ⁎

ns ns ⁎ ⁎⁎⁎

ns o o ⁎⁎⁎

0.0041 0.0095 0.0113 0.0083 0.022 2.9 0.182

ns ⁎⁎ ⁎ ⁎⁎⁎ ns o ⁎⁎⁎

⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ⁎⁎⁎ ns ns ⁎⁎⁎

o ⁎ ns ⁎⁎⁎ ns ⁎ ⁎⁎

ns ns ns ns ns ns ns

⁎⁎⁎ ns ns ns ⁎⁎ ns ns

See Table 3. Energy corrected milk. c Calculated as N output in milk / N intake. d Amino acids absorbed from the small intestine; calculated as [milk protein yield / (AAT intake − AAT maintenance requirements)] ignoring the effects of live weight change. e Calculated as [milk energy / (ME intake − ME maintenance requirements)] ignoring the effects of live weight change. MEDOM intake was determined from intake of digestible organic matter (DOM) assuming ME concentration of 16 MJ/kg DOM. f Calculated as [milk energy / (ME intake − ME maintenance requirements)] ignoring the effects of live weight change. ME content of feeds was calculated using digestibility coefficients determined in vivo with sheep for the silages and feed table values for the concentrates. g Calculated as kg ECM / kg DM intake. b

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MEDOM. No differences in feed conversion efficiency were found between RG silages. Increasing concentrate allowance stimulated higher (P b 0.001) yields of milk, ECM and milk components. Milk fat concentration decreased (P b 0.05), while protein (P b 0.01), lactose and urea (P b 0.05) concentrations in milk as well as LW of the cows increased (P b 0.001) when the amount of concentrate increased. The efficiency of AAT (P b 0.01), MEDOM (P b 0.05) and MEFT (P b 0.01) utilization decreased in response to increased concentrate allowance. 4. Discussion 4.1. The effects of grass growth stage in primary growth The decrease in the silage D-value of 5.0 g/d in PG corresponds well to earlier results (Rinne et al., 1999, Beever et al., 2000; Dawson et al., 2002). The rate of development also depends on the type of grass. For example, the D-value decline of red clover is slower than that of grasses at least in primary growth (Beever et al., 2000; Rinne and Nykänen, 2000), and there may also be important differences between grass species and even varieties in the level and rate of change of digestibility. The effects of a 10 g increase in silage D-value on silage DMI (0.48 kg/d) and ECM production (0.61 kg/d) were higher than the mean values of 0.16 and 0.32 kg/d reported by Rinne (2000) and 0.175 g for silage DMI by Huhtanen et al. (2007a). The high responses in the current experiment may be explained by the relatively low proportion of concentrate in the diet, good fermentation quality of the silages and the high production potential of the cows. The CP intake of the cows decreased clearly with a delay in harvest both in PG and RG resulting in negative values of protein balance in the rumen when the late-cut silages were fed. However, the values of protein balance in the rumen were higher on all diets than the recommended minimum allowance of −20 g/kg DM intake (MTT, 2006), and milk urea concentration exceeded the value of 2.5 mmol/l observed at rumen N equilibrium (Broderick and Huhtanen, 2007). 4.2. The effects of grass growth stage in regrowth The effects of advancing growth on regrowth forage digestibility are quite variable in the literature, which may be explained by the varying environmental conditions during regrowth. Time of previous harvest, annual and seasonal variation in temperature, radiation, water availability etc. are likely to contribute to changes in e.g. the type of regrowth (vegetative vs. generative) and rate

of cell wall lignification (Van Soest, 1994). In some cases the rate of digestibility decline in regrowth has been very slow (Van Soest et al., 1978; Kuoppala et al., 2003), but sometimes similar as in the present experiment (Syrjälä and Ojala, 1978; Gordon 1980; Åman and Lindgren 1983) or even faster (Dawson et al., 2002). Typically the rate of digestibility decline is lower in RG than in PG (Beever et al., 2000) as also observed in the present experiment. This resulted in smaller range in the quality of RG silages than in PG silages although the time difference between the cuts within seasons was similar. Deinum et al. (1968) and Lindberg and Lindgren (1988) found that the digestibility of regrowth grass may change without marked changes in the fibre concentration. In the present data, the increase in NDF concentration with progressing RG was marginal, but the increase in iNDF concentration was substantial showing that not only the concentration but also the quality of the cell wall is important. In contrast to PG, silage DM intake did not decrease in response to a decrease in RG digestibility with progressing growth (EE&LE vs. EL&LL), which may partly be explained by the lower DM concentration and more extensive in-silo fermentation (Wright et al., 2000; Huhtanen et al. 2007a) of the early harvested RG silages. However, ECM production increased 0.29 kg ECM / 10 g increase in silage D-value when cows consumed more digestible RG silages. A difference of 12 d in PG harvest (EE&EL vs. LE&LL) increased average D-value of RG silages only by 13 g/kg DM, but caused an average increase of 0.50 kg/d in silage DM intake and 1.55 kg/d in ECM production. In this case the responses are not confounded with the DM concentration and in-silo fermentation of the silages. The responses to the change in silage digestibility were greater than in PG, but the range in digestibility was small. 4.3. Comparison between the harvests Practical experience and experimental results (Castle and Watson, 1970; Peoples and Gordon, 1989; Heikkilä et al., 1998; Khalili et al., 2005) have suggested inferior intake and production potential of RG silages compared to those made from PG. This effect may partly have been due to analytical problems as at least in vitro pepsin cellulase method overestimates the regrowth silage digestibility, unless separate correction equations are used for PG and RG grass material ((Huhtanen et al., 2006a). In the present experiment, the digestibility of the silages was determined in vivo to minimize the analytical problems.

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The silage and total DM intake, and subsequently ME intake (Fig. 1) and ECM production were higher also in the present data when PG rather than RG diets were fed, but since the average silage D-value was also higher (674 vs. 640 g/kg DM), this was to be expected. When the mean of all four RG silages was compared to L, most of the chemical and digestibility (D-value 640 vs. 644 g/kg DM) parameters were similar the only marked difference being the higher NDF concentration of L. In spite of that, the cows fed L consumed more silage (13.3 vs. 12.5 kg/d) and total DM and produced more milk (32.6 vs. 31.1. kg ECM/d). The higher milk yield may be caused by higher intake of DM and subsequently ME, or improved utilization of ME, or both. The ECM production was closely related to ME intake (R2 = 0.86 for MEDOM and R2 = 0.95 MEFT, n = 12; Fig. 2). This is in agreement with results of Peoples and Gordon (1989) and Heikkilä et al. (1998), who compared spring and autumn silages. They concluded that the lower milk production potential of autumn silages was due to reduced silage DM intake with no difference in the efficiency of energy utilization. The ME intake is influenced by the ME concentration of the feeds and to a large extent also by differences in silage DM intake. Presenting milk production responses in relation to ME intake is more justified than presenting them in relation to silage D-value, i.e. concentration of ME, because many other factors also affect the intake potential of silages. The silage characteristics including digestibility and fermentation quality that affect silage intake were quantified by Huhtanen et al. (2002). Factors such as silage DM concentration and season of harvest may further modify the intake characteristics of silage and an analysis taking also them into account has recently been

Fig. 1. The effect of silage D-value (concentration of digestible organic matter in dry matter) on dietary means of metabolizable energy (ME) intake of dairy cows consuming diets based on primary growth (●) or regrowth (□) grass silages.

179

Fig. 2. The effect of metabolizable energy (ME) intake on dietary means of energy corrected milk (ECM) production of dairy cows consuming diets based on primary growth (●) or regrowth (□) grass silages.

conducted (Huhtanen et al., 2007a).The coefficients of determination (R2) between the old and the new silage DM intake indices and the silage DM intakes observed in the present experiment were 0.75 and 0.98 (n = 6) showing that especially the new index was successful in predicting the intake potential of the current silages. The intake of a RG silage was estimated to be reduced by 0.44 kg DM / d compared with comparable silage prepared from PG based on 46 observations (Huhtanen et al., 2007a). The current data was included in developing the new index. A potential reason for the lower intake of RG silages could be the physical limitation in the rumen. However, the NDF intake of cows consuming RG silages was lower than those fed PG silages. A companion experiment using rumen evacuation technique revealed that the ruminal pools of DM, NDF and iNDF were smaller in cows consuming RG than PG silages suggesting that rumen fill was not limiting silage DM intake (Kuoppala et al., 2004). McRae et al. (1985) reported greater absorption of amino acids from spring than autumn grass. An imbalance between absorbed amino acids and energy could potentially limit feed intake, but the AAT to ME ratios between the harvests in the current experiment (8.1 and 8.3 g AAT / MJ ME for PG and RG) do not support that conclusion. The calculated values may however not reflect the true absorption of nutrients, but according to the companion experiment using fistulated cows, the non-ammonia N flow to the duodenum per DOM intake was similar in PG and RG (unpubl.). The reduced intake potential of RG silages clearly contributes to the lower milk production potential, but the challenge lies in identifying feed factors that cause

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the intake reduction. The microbiological quality of regrowth grass may differ from that of PG as typically weather is more humid and warm later in the summer, and RG grass contained more dead plant material. In the current plant material, the occurrence of leaf spot infections (mainly Bipolaris sorokiniana) was evaluated by weekly sampling for 6 times of both regrowth areas, and the proportion of leaf area destroyed increased with progressing regrowth (data not shown). The fibre concentration is lower at the same digestibility (Peoples and Gordon 1989; Huhtanen et al., 2006) but proportion of iNDF in NDF higher in RG than in PG grass (Huhtanen et al., 2006). The lower digestibility in RG grass cell wall may at least partly be due to the higher mean temperature during the growing time, which stimulates lignification (Van Soest, 1994). The mean temperatures during the growth time of the current silages were clearly lower during PG (on average 12.0 °C) than during RG (17.2 °C). Possibly dynamics of fibre digestion differ in silages harvested in different seasons as the rate of passage of iNDF from the rumen was slower in silages made from RG compared with PG (Kuoppala et al., 2004). The regrowth grass typically has a higher leaf to stem ratio (Beever et al., 2000; Rinne and Nykänen, 2000; Kuoppala et al., 2003; Gustavsson and Martinsson 2004), but at least within harvests, high proportion of leaves is generally associated with higher digestibility and intake potential of grass. Further differences in the grass material in the present experiment included higher proportion of timothy and lower proportion of other plant species in PG compared to RG, but it is difficult to estimate the significance of these factors. 4.4. Responses to increased concentrate allowance The mean response of 0.62 kg ECM per kg increase in concentrate DM intake with diets based on PG was in agreement with earlier results (Huhtanen, 1998). The ECM response increased with increasing maturity of grass ensiled in PG being 0.34 and 1.01 kg ECM/kg additional concentrate DM for E and L, respectively. Rinne et al. (1999) could not detect an interaction between silage digestibility and level of concentrate supplementation, but benefits of high digestibility silages were reduced as the concentrate proportion increased in the study of Ferris et al. (1999). The effect is mainly mediated by changes in total DMI. The greater substitution rate when high rather than low digestibility silages has been suggested (Thomas, 1987) but could not be detected in an analysis of a large data set (n = 217 treatment means; Huhtanen et al., 2007b). However, the

higher the silage DM intake index of silages (Huhtanen et al., 2007a), the greater the SR was (Huhtanen et al., 2007b) suggesting that SR is more closely related to the overall intake potential of silage than to D-value alone. The ECM response to increased concentrate allowance was greater (0.92 kg/kg concentrate) when RG rather than PG silages were fed. The intermediate substitution rates of 0.53, 0.43, 0.42 and 0.54 for the RG silages EE, EL, LE and LL compared to PG cannot readily be related to the digestibility of the silages, but ECM responses were greater for the late rather than the early-cut RG silages similarly as in PG. In a large data set (Huhtanen et al., 2007b), no differences in the SR of PG and RG silages could be detected (0.48 v. 0.47). The average marginal ECM response to increased MEFT intake achieved by increased concentrate allowance was slightly lower in PG than RG (0.09 vs. 0.12 kg ECM per 1 MJ increase in MEFT). The responses were similar to that (0.10 kg ECM per MJ) observed by Rinne et al. (1999). The responses to increased ME intake may vary if MEDOM is used instead of MEFT, because the predicted ME intakes are not always realised as expected. The MEFT intake was calculated to increase by 24.1 MJ in PG in response to increased concentrate supplementation but MEDOM intake increased only by 11.9 MJ. The difference between the methods was smaller in RG (25.5 and 21.2 MJ, respectively). The reasons contributing to the difference in MEDOM and MEFT include negative associative effects on diet digestibility, which were smaller in RG evidenced by a significant (P b 0.05) interaction in diet OMD between level of concentrate supplementation and season. The changes in milk composition, i.e. an increase in protein concentration and a decrease in fat concentration in response to increased concentrate allowance, were typical (Huhtanen, 1998). There were no statistically significant interactions between the level of concentrate supplementation and season of silage harvest or harvest time within seasons. 5. Conclusions Milk production of dairy cows reflected the intake of ME and no differences in the utilization of ME were found between silages harvested from PG and RG. However, intake of RG silages was slightly lower than that of comparable PG silage, but definite reasons for that were difficult to identify. Practical implications of the results depend on whether the aim is to maximise the production per cow or per hectare of grass. Optimal harvesting strategy should be chosen based on the individual limitations of a particular farming situation.

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