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Effects of Varying Lactation Length on Milk Production Capacity of Cows in Pasture-Based Dairying Systems

M. J. Auldist^*,1, G. O’Brien^*, D. Cole^*, K. L. Macmillan and C. Grainger^*

^* Department of Primary Industries, Ellinbank, Victoria 3821, Australia
Department of Veterinary Science, University of Melbourne, Werribee, Victoria 3030, Australia

¹ Corresponding author: vfmeditor{at}fishingmonthly.com.au

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

 
The aim of this experiment was to quantify the milk production capacity of cows undergoing extended lactations while fed a pasture-based diet typical of those used in the seasonal-calving dairying systems of Victoria, Australia. One hundred twenty-five Holstein cows were randomly assigned to 1 of 5 groups. Breeding was progressively delayed after calving to enable management of the groups for lactation lengths of 10, 13, 16, 19, and 22 mo (equivalent to calving intervals of 12 to 24 mo). Cows were provided with a daily energy intake of at least 180 MJ of metabolizable energy/cow. This was supplied primarily by grazed pasture with supplementary cereal grain, pasture silage, and hay. Cows were dried off when milk volume fell below 30 kg/wk or when they reached 56 d before their expected calving date. Most cows (>96%) could lactate above this threshold for 16 mo, >80% for 19 mo, and >40% for 22 mo. There were negative relationships between lactation length and annual production of milk and milk solids (milk fat + protein), but losses were small until 16 mo. Annualized yields of milk solids were 497, 498, 495, 474, and 463 kg/cow for the 10, 13, 16, 19, and 22 mo groups, respectively. This reduction in annual production of milk solids with increasing lactation length was relatively less than for milk volume because during extended lactation, cows produced milk with higher concentrations of protein. Cows undergoing extended lactations also finished their lactations having gained more body weight and body condition than cows lactating for only 10 mo. The data showed that many cows on pasture-based diets were capable of lactating longer than the 10 mo that is standard for Victorian herds with seasonally concentrated calving patterns. Further, such extended lactations could be achieved with little penalty in terms of annual milk solids production.

Key Words: extended lactation • milk production • pasture • protein

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

 
In the pasture-based dairy industry in Victoria, Australia, cows have traditionally been managed to calve in late winter. Then, after a 10-mo lactation they are dried off for a period of 8 to 10 wk. This seasonally concentrated calving system has been used to maximize pasture utilization. Several changes in the Victorian dairy industry have recently prompted a reassessment of whether such systems are the most appropriate.

The increase in the use of semen from Holstein sires since 1970 has been associated with a general decline in the reproductive performance of Victorian herds (Morton, 1999). The Holstein breed has a higher milk yield potential than other breeds and undergoes greater weight losses in early lactation when body adipose reserves are mobilized to sustain these yields (Fulkerson et al., 2001; Snijders et al., 2001). These factors have been linked to lower conception and pregnancy rates (Beever et al., 2001; Lucy, 2001). Low reproductive performance in a seasonally concentrated calving system is exacerbated by insufficient time to recover from the negative energy balance of early lactation, because the required time of breeding coincides with peak lactation (Borman et al., 2004). As a consequence, herd owners can have difficulty maintaining a 12-mo calving interval. There is evidence that this has occurred in Victoria: between 2004 and 2006 the proportion of seasonal-calving herds in Victoria declined from 63 to 41% (Dairy Australia, 2006).

Attempts to maintain a seasonally concentrated calving system in combination with poor conception rates lead to high numbers of cows being removed from the herd because they were not pregnant (Evans et al., 2006). Additionally, there is increasing reliance on the induction of premature calving to maximize the duration between calving and breeding (Morton, 1999). Systems that incorporate lactations longer than the traditional 10 mo may alleviate these issues by providing cows with more time to conceive after calving and peak lactation (Schindler et al., 1991; Ratnayake et al., 1998; Larsson and Berglund, 2000). Overseas studies have shown that cows are capable of producing milk for significantly longer than 10 mo (Van Amburgh et al., 1997; Osterman and Bertilsson, 2003; Kolver et al., 2006). Similarly, cows in commercial herds in Victoria are known to have lactated for 22 mo or more (M. J. Auldist, unpublished data). There are, however, no controlled studies demonstrating the potential for longer lactations among Victorian cows managed in pasture-based systems.

The aim of this experiment was to determine the effect of lactation length on production of milk, milk fat, and milk protein in grazing cows milked for between 10 and 22 mo.

	MATERIALS AND METHODS

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Cows and Design
One hundred twenty-five AI-bred Holstein cows that calved in July 2003 were selected from the research herd maintained at DPI Ellinbank (latitude 38°14'36.4'', longitude 145°56'09.5''). Using the method described by Baird (1994), cows were assigned randomly to 1 of 5 groups of 25 (each group included 5 primiparous cows) with each group balanced for age of cow, calving date, BW, and yields of milk, protein, and fat in the preceding lactation (excepting primiparous cows).

One group of 25 cows was assigned to each of 5 designated lactation length targets of 10, 13, 16, 19, and 22 mo. These were equivalent to calving intervals of 12, 15, 18, 21, and 24 mo, respectively (allowing for a 56-d intercalving dry period). The breeding program for each group was initiated approximately 7 mo before the date at which its designated lactation length target would be met.

Control Herd
The group managed for a 10-mo lactation acted as a control. Cows in this group had two 10-mo lactations during the 2-yr duration of the experiment (separated by a 56-d dry period). Cows that failed to conceive in the first year were replaced by similar cows for the second lactation, including 5 replacement primiparous cows.

Management
All lactating cows were grazed together as a single herd and were subjected to the same management practices with the exception of the timing of their breeding program. Cows grazed on a dedicated series of paddocks distributed around the research farm at DPI Ellinbank. They were milked twice daily through a common parlor at approximately 0700 and 1500 h.

Nutrition for lactating cows was kept as similar as possible throughout the experiment by providing an estimated minimum daily dietary intake of 180 MJ of ME/cow. It was calculated that this level of intake would enable cows to produce 280 kg of milk fat in a 10-mo lactation (Standing Committee on Agriculture, 1990). This was slightly higher than the 2004–2005 Victorian average of 259 kg of milk fat for Holstein cows in herd-recording herds (National Herd Improvement Association, 2005). This energy was provided primarily by grazed pasture. When pasture availability or quality was limited, the balance of the 180 MJ of ME was met with cereal grain, pasture silage, and legume hay. Cows were allowed to consume more than 180 MJ of ME/d from pasture when it was available.

Because of the inherent difficulty in measuring how much pasture the cows consumed each day, their energy intake was back-calculated at regular intervals using equations based on known energy requirements for the prevailing levels of milk production, BW, and BW change (Standing Committee on Agriculture, 1990). In accordance with the calculated ME intake, the diet was then adjusted as necessary to ensure that 180 MJ of ME was being provided. This involved adjusting the pasture allowance (using electric fencing) and varying the supplementary grain and forage. The pasture intake of cows was supplemented with 2 to 7 kg DM/d of grain (mostly barley: a total of 1.6 t/cow in the first 10 mo), up to 9 kg DM/d of hay (pasture or legume), or up to 5 kg DM/d of pasture silage at different times. The concentrations of CP and fiber and the apparent digestible DM of pasture and other dietary components are presented in Table 1.

Breeding Program
All cows in the experiment were enrolled in a breeding program that involved the synchronization of estrus and 3 rounds of AI. The program commenced 10 d before the planned start of mating for each group (determined by target lactation length) at which time every cow not previously observed in estrus was physically examined by a veterinarian to determine whether they were anestrous. All cows then had a controlled internal drug-releasing (CIDR) device (Pfizer Australia, West Ryde, NSW, Australia) inserted vaginally and received an i.m. injection of estradiol benzoate (1 mg for anestrous cows, 2 mg for cycling cows). Each device was removed 8 d after insertion at which time all cows received an injection of prostaglandin F₂ (Lutalyse, Pfizer Australia) and had an estrus detection aid applied. Estruses occurred from 2 d after CIDR removal and cows were artificially inseminated to detected estrus by a trained technician.

Three weeks after initial CIDR insertion, devices were reinserted into each inseminated cow and each cow received an injection of 1 mg of estradiol benzoate. The program was repeated and resynchronized cows were again inseminated to detected estrus. The program was repeated a third and final time for those cows inseminated during the second round. Subsequently, any cows still not pregnant were inseminated to observed estrus.

Dry-Off Threshold
Cows that had an average milk yield of less than 30 kg/wk for 2 consecutive weeks were dried off irrespective of whether they had met their lactation target. Those cows that maintained production above this level for the entire lactation were dried off 56 d before their expected calving date. The dry-off threshold was deliberately set at a low level to ensure cows were not removed from the experiment prematurely (e.g., some cows undergoing extended lactations are capable of recovering from low production levels when pasture quality improves in spring; Kolver et al., 2006).

Measurements and Analyses
Milk yield was measured daily for each cow using a DeLaval Alpro milk metering system (DeLaval International, Tumba, Sweden), with the exception of a 12-wk period at the start of the trial when it was measured every 2 wk using Tru-Test milk meters (Tru-Test Ltd., Auckland, New Zealand). A composite sample was taken of the daily milk (p.m. and a.m.) from each cow using inline milk meters once every 2 wk and tested for concentrations of fat, protein, and lactose using an infrared milk analyzer (model 2000, Bentley Instruments, Chaska, MN).

Cows were weighed approximately monthly, at which time their BCS was also assessed. Samples of pasture and other feeds offered to cows were collected once every 2 wk. These feed samples were oven dried, ground through a 0.5-mm sieve, and then analyzed for composition by near-infrared spectroscopy by a commercial laboratory (FeedTest, Hamilton, Victoria, Australia).

Statistical Analyses
The data were used to calculate total lactation yields for each cow. The log-transformed data were then subjected to analysis using an additive mixed model with fixed effects for lactation length [6 levels: 10 mo (A), 10 mo (B), 13 mo, 16 mo, 19 mo, and 22 mo] and for age (2 levels: heifer and cow). The 10 mo (A) and (B) groups were the continuation of the same herd in 2 successive lactations, but with necessary replacement cows. The mixed model specified random effects for cow and repeated measures within cow. The analysis provided a vector of treatment group mean estimates adjusted for age group and where relevant (10-mo groups) for cow, and a corresponding variance-covariance matrix, as well as Wald significance tests for differences between groups (and between age groups). Graphs of residuals vs. fitted values were used to check constant variance, and histograms and q-q plots of residuals were used to check the normal distribution assumption. The log-transformation was applied to meet these statistical requirements.

Group means were "annualized" to estimate the production on a per-year basis and thus more fairly compare the levels of production of the different lactation lengths over time. This was achieved by multiplying each mean by 12/CI, where CI was the calving interval for the treatment in months; namely, the specified lactation length + 2. This was equivalent to, and was effected by, adding log(12/CI) to the means on the log scale.

An attempt was made to address the fact that treatments were measured over different periods of time by comparing the different groups with a control (10-mo lactation) estimate that as far as possible represented the period over which the lactation length of interest was measured. For each lactation length group, a reference control estimate was therefore defined as a weighted mean of the 2 available control groups (10-mo A and 10-mo B), with the weighting apportioned as w = 11/(lactation length), and 1 – w respectively. For example, for the 22 mo group, w = 11/22 = 0.5. The reference control mean for this treatment was the un-weighted average of the 10-mo (A) and (B) group estimates (using estimates from the mixed model analysis). For the 13-mo group, w = 11/13 = 0.846, and the reference control was therefore 85% of the 10-mo (A) group estimate plus 15% of the 10-mo (B) group estimate.

These results have been further adjusted to present a single control estimate (while retaining the lactation length treatment to reference-control differences). Each of the above adjustments to treatment means was achieved by linear transformations applied to the vector of group estimates and its corresponding quadratic product with variance-covariance matrix, saved from the mixed model analysis.

Linear and quadratic orthogonal polynomial contrasts were applied to the adjusted treatment means to test for relationships with lactation length. The adjusted treatment means and their variance covariance matrix were also used to compute 5% least significant intervals (Snee, 1981) for the treatment means. These were back-transformed (antilogged) to the original scale for presentation. Pairs of means with nonoverlapping least significant intervals were significantly different at the P < 0.05 level.

Fisher’s exact test was used to compare the proportions of cows reaching target lactation length.

	RESULTS

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Proportion of Cows Reaching Their Target Lactation Length
Proportions of cows reaching their target lactation length are presented in Table 2 together with average DIM for each group. At least 96% of all cows were able to maintain milk weight greater than the designated dry-off threshold of 30 kg/wk for target lactation lengths up to and including 16 mo. Only 83% (20/24) and 42% (10/24) of cows were able to sustain their milk production above the required amount for 19- and 22-mo lactations. The proportion of cows reaching 19 and 22 mo was less (P < 0.05) than for 10 to 16 mo.

View larger version (14K): [in this window] [in a new window]   Figure 1. Lactation curves for production of milk and milk solids (protein + fat) for cows undergoing lactations of 10 (2 lactations: ), 13 (), 16 (), 19 (•), or 22 mo (). Data are means for all lactating cows; arrows indicate time of dry off for the control herd in their first lactation.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

Dairy herd owners in Australia and New Zealand receive payment for milk on the basis of the quantity of protein and fat, with a penalty for milk volume. As a consequence, the term "milk solids" (representing the quantities of protein + fat) is widely regarded as a more important production parameter than milk volume. In this experiment there was an increase in both the amount of milk and milk solids produced as lactation length increased, primarily because of additional DIM (Table 3). A more meaningful estimate by which to compare the production of extended lactation systems is to use annualized production figures (Table 4) that take into account the fact that, compared with traditional seasonal-calving systems, cows with extended lactations spend less time in peak production but also more time lactating.

The current experiment showed a negative relationship between lactation length and the annualized production of milk and milk solids (Table 6). Nevertheless, there were few significant differences between individual treatment means for the different lactation length groups (Table 4). Extending lactation from 10 to 16 mo resulted in a numerical milk solids loss of <2 kg/cow per year. This amount of loss was much less than the 31-kg loss reported by Osterman and Bertilsson (2003) for cows producing 616 kg of milk solids per year. Even when lactation length was extended to 22 mo the loss of milk solids in the current experiment was only 32 kg/yr (around 5%). This was similar to the milk solids loss of 6% reported by Kolver et al. (2006) for cows that were genetically of North American origin grazing pasture in New Zealand and offered a similar level of supplements as the cows in the current experiment.

Cows with extended lactations had higher average concentrations of milk protein compared with cows in the control group undergoing normal lactations of 10 mo (Table 3). This was one reason why the loss of milk solids with increasing lactation length was smaller than the loss of milk volume. This increase in milk protein concentration was because of elevated protein during the extended lactation component of the lactation (after 300 d). This phenomenon has been observed among individual cows completing lactations of >1,000 d in commercial Victorian dairy herds (M. J. Auldist, unpublished data), and for cows in New Zealand undertaking 600-d lactations (Kolver et al., 2006).

Cows in the longer lactation groups completed their lactations with greater BW and BCS compared with cows undergoing 10-mo lactations (Table 5). This was also observed by Kolver et al. (2006) and would be because of cows spending more time in positive energy balance after peak milk production. Higher BCS at calving can increase production of cows in the ensuing lactation (Grainger et al., 1982; Pryce and Harris, 2006), although the effect of consecutive extended lactations remains untested within grazing dairy systems.

In summary, the current experiment has shown that cows managed under a pasture-based system were able to sustain extended lactations with good persistency, even up to 22 mo. Compared with standard 10-mo lactations, there were small annual losses of milk solids yield during extended lactations. Cows undergoing extended lactations had greater milk protein concentrations and gained more body condition and BW during their lactation than control cows. The data indicate a potential role for extended lactations in Victorian pasture-based dairying systems in which Holstein dairy cattle now predominate.

	ACKNOWLEDGEMENTS

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The authors are grateful to D. Mapleson, I. Robinson, R. Case, T. Hookey, and staff at the DPI Ellinbank research farm for technical assistance with cow husbandry, grazing management, and sampling procedures, and to M. Hannah for conducting the statistical analyses. This research was funded by the Department of Primary Industries (Victoria) and the Geoffrey Gardiner Dairy Foundation Limited.

Received for publication October 18, 2006. Accepted for publication February 22, 2007.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

 


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