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* Dexcel, Hamilton, New Zealand
Teagasc Moorepark, Fermoy, County Cork, Ireland
1 Corresponding author: john.roche{at}utas.edu.au
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ABSTRACT |
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 TOP ABSTRACT ACKNOWLEDGEMENTSREFERENCES |
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Key Words: genetic merit • ghrelin • dry matter intake • milk production
The initiation of lactation dramatically alters the metabolism of many organs so that the mammary gland can be supplied with the nutrients necessary for milk synthesis (Bauman and Currie, 1980). Few factors could be regarded as more important in this homeorhetic process than the hyperphagia associated with early lactation.
Consistent with this, genetic selection for milk production has resulted in increased DMI (Kennedy et al., 2003; Linnane et al., 2004; Kolver et al., 2005), probably as a result of either associated genes being closely linked or pleiotropic gene effects (i.e., genes that affect milk production also affect DMI; Veerkamp et al., 2003). However, this pleiotropy may also extend to increased body tissue mobilization in early lactation and a reduced propensity to partition nutrients toward body tissue replenishment during lactation (Roche et al., 2006). Greater negative energy balance (NEBAL) is undesirable because of associated negative effects on reproductive performance in dairy cows (Beam and Butler, 1999; Buckley et al., 2003) and a consequential reduction in the profitability of grazing systems (Veerkamp et al., 2002). The conundrum that therefore exists is how to continue selection for increases in DMI without exacerbating the already problematic NEBAL.
One possibility may be in genetic selection for markers of ghrelin production, if such a trait is related to increased DMI in dairy cows. Ghrelin is one of the most powerful peripherally active orexigenic agents known (Wren et al., 2001). Although initially identified as a growth hormone secretagogue (Kojima et al., 1999), ghrelin has since been reported to have non-growth hormone-related effects (Korbonits and Grossman, 2004), stimulating DMI, body growth and adiposity, and milk production in rodent models (Tschop et al., 2000; Nakahara et al., 2003). It has also been associated with the regulation of other endocrine functions, such as prolactin, cortisol, and luteinizing hormone (Korbonits and Grossman, 2004). However, there is limited information about this hormone in lactating dairy cows (Robinson et al., 2006). The objective of this study was to determine whether genetic selection for increased milk production was associated with increased plasma concentrations of ghrelin.
Twenty-one low genetic merit (LGM; n = 12) and high genetic merit (HGM; n = 9) Holstein-Friesian cows in peak lactation were offered unrestricted access to fresh pasture. Blood samples were taken prior to the a.m. milking (i.e., baseline) and following 2 h of grazing after the a.m. milking on 2 consecutive wk during peak lactation.
The 2 strains were selected to be divergent in EBV for milk production, but were balanced for age and DIM (5.5 ± 0.67 and 4.7 ± 1.32 yr, and 82 ± 16.0 and 65 ± 17.2 DIM, for LGM and HGM, respectively). The mean EBV were obtained from the Livestock Improvement Corporation, New Zealand, evaluations (December 2005). The mean EBV for the LGM animals under study were +412 (SD 242.1) kg of milk, +13 (SD 7.3) kg of fat, +7 (SD 4.7) kg of protein, and +41 (SD 15.6) kg of BW, whereas the EBV for the HGM cows were +806 (SD 224.7) kg of milk, +32 (SD 7.3) kg of fat, +31 (SD 4.7) kg of protein, and +55 (SD 15.6) kg of BW.
Grazing management was similar to that described by Roche et al. (2006). Briefly, cows were rotationally grazed as one herd and were assigned a grazing area only when a minimum of 2 leaves had appeared on the majority (>75%) of perennial ryegrass tillers. Cows had access to a new allocation of pasture daily. Pasture allowance (>40 kg of DM/cow per d) was sufficient to ensure unrestricted DMI (up to approximately 25 kg of DM/d) of fresh pasture. Despite the high grazing residuals, quality was maintained (Table 1
) through strategic use of mowing following grazing.
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Individual milk yields were recorded daily (West-falia Surge, Oelde, Germany). Fat, CP, and lactose concentrations of milk were determined by Milkoscan (Foss Electric, Hillerød, Denmark) on individual p.m. and a.m. aliquot samples, with the a.m. samples coinciding with the same day as blood sampling. Milk component data were verified by reference techniques for a subset of milk samples (milk fat, Röese-Gottlieb technique; CP, Kjeldahl technique). Body weight was determined weekly following the a.m. milking.
Mean DMI was calculated from milk energy output and cow maintenance requirements, accounting positively or negatively for BW gain or loss, respectively, as described by Roche et al. (2005). Energy intake was divided by the mean pasture ME concentration to calculate DMI:
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Two evacuated blood tubes, (140 IU sodium heparin and 0.117 mL of 15% K3EDTA) were collected from each cow by coccygeal venipuncture prior to the a.m. milking and 2 h following the return to pasture, and plasma was extracted (1,120 x g, 10 min, 4°C). Plasma samples from the EDTA-blood tubes were acidified using 0.1 N HCl and treated with phenylmethylsulfonyl fluoride (C7H7FO2S), following kit instructions (Ghrelin Active Kit; Linco, St. Charles, MO, cat. no. GHRA-88HK), prior to storage and analysis for plasma ghrelin concentration.
Analyses for NEFA (colorimetric method) and glucose (hexakinase method) were performed on a Hitachi 717 analyzer (Roche, Basel, Switzerland) at 30°C by Alpha Scientific Ltd. (Hamilton, New Zealand). The inter- and intra-assay coefficient of variation was <2% for NEFA and glucose. Growth hormone and ghrelin were measured in duplicate by double-antibody radio-immunoassay with an inter- and intra-assay coefficient of variation <10%.
A total of 42 prefeeding and 42 postfeeding records of plasma metabolite and hormone concentrations were available for analysis. Preliminary analysis of all data revealed a positively skewed distribution for plasma ghrelin, growth hormone, and NEFA concentration. The natural logarithm of all 3 variables was used to normalize the distribution; the Shapiro–Wilk test signified a normal distribution following transformation.
Mixed-model methodology using the PROC MIXED procedure (SAS Institute, 2006) was used to investigate the effects of genetic merit, parity, and time of measurement (i.e., pre- or postfeeding) on plasma ghrelin, growth hormone, NEFA, and glucose concentrations using all data. Cow was included as a random effect. A full model was initially created with all main effects and interactions included as classification variables, and DIM at sampling was included as a continuous variable. Terms were sequentially removed from the model using backward elimination of nonsignificant (P > 0.05) effects based on the F-test. Least squares means were extracted from the analysis. The main effects of genetic merit, time of sampling, and parity were always forced into the model to facilitate the estimation of the respective least squares means. Mixed-model analyses were also undertaken to determine the effect of genetic merit, after adjusting for parity and DIM, on milk production and DMI.
As predicted from the EBV for milk production, HGM cows produced more (P < 0.05) milk than LGM cows (26.9 vs. 22.8 kg, respectively). Mean fat, protein, and lactose contents were not affected by genetic merit (4.29 vs. 4.63%, 3.23 vs. 3.38%, and 4.95 vs. 4.89% of fat, protein, and lactose for HGM and LGM, respectively), but component yield was greater (P < 0.05) in HGM cows. Change in BW over the measurement period did not differ significantly between genetic merit groups (0.78 vs. 0.88 kg/d for HGM and LGM, respectively), suggesting that the difference in milk production at peak lactation in the current study was primarily a result of the difference in pasture DMI (P < 0.05; 17.1 vs. 15.8 kg of DM/cow per d for HGM and LGM, respectively). Kennedy et al. (2003) and Linnane et al. (2004) also reported higher DMI in HGM cows, and greater responses to supplements in HGM cows have been associated with less substitution of supplements for pasture (Kennedy et al., 2002; Kolver et al., 2005), presumably because of a greater drive to produce milk and hence eat.
Plasma ghrelin and growth hormone concentrations were greater (P < 0.05) in HGM cows pre- and postgrazing, and no significant interaction between cow genetic merit and time of sampling was evident (Table 2). Genetic merit did not affect the plasma NEFA or glucose concentration. Plasma concentrations of all metabolites and hormones measured were diminished (P < 0.001) 2 h after feeding. Days in milk did not significantly affect plasma ghrelin and growth hormone concentrations, although it did significantly affect NEFA and glucose concentrations. However, the range in DIM (mean = 81 d; SD = 19 d) was narrow in the present study.
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Itoh et al. (2005) reported declining ghrelin concentrations as cows went from early to mid and late lactation, suggesting greater ghrelin production in cows in greater NEBAL. In the present study, DIM did not significantly affect plasma ghrelin concentration, although only a limited range in DIM (i.e., 47 to 109 DIM) was represented. However, ghrelin has been positively associated with hunger score in humans (Cummings et al., 2004), indicating a greater appetite drive with increasing plasma ghrelin concentrations. The positive effect of ghrelin on DMI and milk production in rodent models is consistent with the greater DMI and milk production in HGM compared with LGM cows in the current study, and the associated trends in plasma ghrelin concentration. These findings suggest a greater appetite in HGM than LGM cows, a result consistent with those of Kennedy et al. (2003), Linnane et al. (2004) and Kolver et al. (2005). Linnane et al. (2004) reported no effect of genetic strain on either mean total grazing time or the number of daily grazing events, but reported a 6% increase in the biting rate (bites/min) in HGM compared with LGM cows in their study. These data indicate a more voracious appetite in HGM cows, consistent with the preprandial plasma ghrelin concentrations reported here (301 vs. 241 pg/ mL in HGM and LGM cows, respectively), and the increased appetite score associated with elevated ghrelin concentrations in humans (Cummings et al., 2004).
The higher postprandial plasma ghrelin concentration in HGM cows (173 vs. 127 pg/mL in HGM and LGM cows, respectively) is also consistent with the reduced negative impact of supplementary feeds in HGM cows compared with lower-merit comparisons (Kennedy et al., 2002; Kolver et al., 2005), with greater ghrelin concentrations postprandially being indicative of a stronger residual appetite. The results presented by Linnane et al. (2004) concur, with LGM cows having a reduced biting rate when supplemented with concentrates but with no effect of concentrate supplementation on biting rate in cows of HGM for milk production.
Results suggest an increase in plasma ghrelin associated with genetic selection for milk production, and an associated increase in DMI. Future research should attempt to quantify the usefulness of plasma ghrelin concentration as a genetic predictor of DMI.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTACKNOWLEDGEMENTSREFERENCES |
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FOOTNOTES |
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Received for publication March 19, 2006. Accepted for publication April 3, 2006.
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REFERENCES |
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TOPABSTRACTACKNOWLEDGEMENTSREFERENCES |
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This article has been cited by other articles:
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  J. R. Roche, A. J. Sheahan, L. M. Chagas, and D. P. Berry Concentrate Supplementation Reduces Postprandial Plasma Ghrelin in Grazing Dairy Cows: A Possible Neuroendocrine Basis for Reduced Pasture Intake in Supplemented Cows J Dairy Sci, March 1, 2007; 90(3): 1354 - 1363. [Abstract] [Full Text] [PDF]
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